1499 16. Vejerslev LO, Mikkelsen M. The European collaborative study on mosaicism in chorionic villus sampling: data from 1986 to 1987. Prenat Diag 1989; 9: 575-88. 17. Ledbetter DH, Martin AO, Verlinsky Y, et al. Cytogenetic results of chorionic villus sampling: high success rate and diagnostic accuracy in the United States collaborative study. Am J Obstet Gynecol 1990; 162: 495-501. 18. Schulze B, Miller K. Chromosomal mosaicism and maternal cell contamination in chorionic villus cultures. Clin Genet 1986; 30: 239-40. 19. Roberts E, Duckett DP, Lang GD. Maternal cell contamination in chorionic villus samples assessed by direct preparations and three different culture methods. Prenat Diag 1988; 8: 635-40. 20. Association of Clinical Cytogeneticists Working Party. A proposed workload measurement system for clinical genetics. Association of Clinical Cytogeneticists, London, 1987. 21. Tabor A, Madsen M, Obel EB, et al. Randomised controlled trial of genetic amniocentesis in 4606 low-risk women. Lancet 1986; i: 1287-93. 22. Green JM. Prenatal screening and diagnosis: some psychological and social issues. Br J Obstet Gynaecol 1990; 97: 1074-76. 23. Feeny DH, Torrance GW. Incorporating utility-based quality-of-life assessment measures in clinical trials. Medical Care 1989; 27: S190-204. 24. Bryce RL, Bradley MT, McCormick SM. To what extent would women prefer chorionic villus sampling to amniocentesis for prenatal diagnosis? Paediatr Perinat Epidemiol 1989; 3: 137-45. 25. Rooney DE, MacLachan N, Smith J, et al. Early amniocentesis: a cytogenetic evaluation. Br Med J 1989; 299: 25. 26. Penso CA, Frigoletto FD. Early amniocentesis. Semin Perinatol 1990; 14: 465-70. The effect of insecticide-treated bed nets on mortality of Gambian children Insecticide treatment of bed nets ("mosquito nets") may be a cheap and acceptable method of reducing the morbidity and mortality caused by malaria. In a rural area of The Gambia, bed nets in villages participating in a primary health-care (PHC) scheme were treated with permethrin at the beginning of the malaria transmission season. Additionally, children aged 6 months to 5 years were randomised to receive weekly either chemoprophylaxis with maloprim or a placebo throughout the malaria transmission season. We measured mortality in children in PHC villages before and after the interventions described, and compared this with mortality in villages where no interventions occurred (non-PHC villages). About 92% of children in PHC villages slept under insecticide-treated bed nets. In the year before intervention, mortality in children aged 1-4 years was lower in non-PHC villages. After intervention, the overall mortality and mortality attributable to malaria of children aged 1-4 in the intervention villages was 37% and 30%, respectively, of that in the non-PHC villages. Among children who slept under treated nets, we found no evidence of an additional benefit of chemoprophylaxis in preventing deaths. Insecticide-treated bed nets are simple to introduce and can reduce mortality from malaria. Introduction In rural areas of The Gambia, 4s in other parts of tropical Africa, malaria remains one of the most important causes of death in children under the age of 5 years.1 Until a cheap, safe, and effective vaccine becomes available, malaria control in such communities will rely primarily on antimalarial drugs for treatment and prevention, and on the reduction of man-vector contact. Insecticide-treated bed nets ("mosquito nets") may be a relatively cheap and acceptable method of reducing man- vector contact.2.3 However, the efficacy of insecticide- treated materials as a form of malaria control is controversial. Results of some previous studies are difficult to interpret and are complicated by variations in the epidemiology of malaria in the different areas where trials have been done. Moreover, comparison between the few studies that have attempted to evaluate the effect on morbidity of insecticide- treated material has been made difficult by the use of different clinical and parasitological measurements.’ No studies have shown an effect of insecticide-impregnated bed nets on mortality. Studies in the Farafenni area of The Gambia showed that morbidity from malaria was reduced by targeted chemoprophylaxis with maloprim (pyrimethamine and dapsone) and the use of permethrin-impregnated bed nets, and that each of these malaria-control strategies can be used in a village-based primary health-care (PHC) scheme.S.6 Therefore, we have done a field study to determine whether insecticide treatment of bed nets could be implemented on a large scale as a malaria-control strategy, and we report the impact of this intervention on mortality in young children. In addition, we report the effect on mortality of targeted chemoprophylaxis in children already protected by insecticide-impregnated bed nets. Subjects and methods Study area and study population The trial was done in 73 villages on the south bank of the river Gambia, east of the town of Soma and about 200 km from the Atlantic coast. The area is one of flat Sudan savanna, with mangrove swamps bordering the river which is still partly saline at this point. The climate is characteristic of the sub-Sahel, with a long dry season ADDRESSES: Medical Research Council Laboratories, Banjul, The Gambia (P. L. Alonso, MD, S. W. Lindsay, PhD, J. R. M. Armstrong, MSc, A. de Francisco, MD, F. C. Shenton, MSc, B. M. Greenwood, MD); Medical and Health Department, Government of The Gambia (M. Conteh, CHN, K. Cham, MD); Centre for Population Studies, London School of Hygiene and Tropical Medicine, London, UK (A. G. Hill, PhD, P. H. David, MSc, G. Fegan, MSc); and The Gambia Hepatitis Intervention Study, International Agency for Research on Cancer, Lyon, France (A. J. Hall, MRCP). Correspondence to: Dr P. L. Alonso, Institute de Parasitologia "Lopez Neyra", Ventanilla, 11. 18001 Granada, Spain.