380 Sexually transmitted diseases (STDs) are responsible for significant morbidity and mortality worldwide be- cause of their adverse effects on reproductive health and their ability to increase the risk of sexual and vertical transmission of human immunodeficiency virus type 1 (HIV-1). 1 More than 333 million new cases of the 4 most common curable STDs (gonorrhea, chlamydia, tricho- moniasis, and syphilis) occur annually, most in the devel- oping world. 2 STDs, even when excluding HIV-1, are the second leading cause of lost healthy life years in women of reproductive age in developing countries. 3 Identifica- tion of risk factors for acquisition of STDs is of global public health importance. Hormonal contraceptives, including oral contraceptive pills and the long-acting injectable progestin depot me- droxyprogesterone acetate (DMPA), are used by more than 100 million women worldwide. 4 The relationship between oral contraceptive pill use and STD acquisition has been a topic of investigation for many years, though methodological problems (eg, cross-sectional design and failure to control for potential confounding factors such as sexual behavior) have hampered many studies. 5-7 Con- sistently higher rates of cervical infection with Chlamydia trachomatis have been found among users of oral contra- ceptive pills, 6 but the relationship with other STDs is un- clear. 8 Very little information regarding the risk of STD acquisition in women who use injectable progestin con- traception has been reported. 8 From the Departments of Epidemiology, Biostatistics, and Medicine, Uni- versity of Washington a (H.L.M. is now with Park Nicollet Clinic, Min- neapolis, Minnesota); the Department of Medical Microbiology, Univer- sity of Nairobi b ; and Coast Provincial General Hospital. c This study was conducted in Mombasa, Kenya. †Deceased. Supported in part by US National Institutes of Health grants AI33873, D43-TW00007, and T22-TW00001 and by Family Health Interna- tional (subcontract NO1-A1-35173-119). Received for publication October 5, 2000; revised February 21, 2001; ac- cepted March 19, 2001. Reprint requests: Jared M. Baeten, University of Washington, 325 Ninth Avenue, Box 359909, Seattle, WA 98104-2499. E-mail: jbaeten@ u.washington.edu. Copyright © 2001 by Mosby, Inc 0002-9378/2001 $35.00 + 0 6/1/115862 doi:10.1067/mob.2001.115862 Hormonal contraception and risk of sexually transmitted disease acquisition: Results from a prospective study Jared M. Baeten, BA, a Patrick M. Nyange, MBChB, MPH†, b Barbra A. Richardson, PhD, a Ludo Lavreys, MD, MSc, a Bhavna Chohan, MS, b Harold L. Martin, Jr, MD, MPH, a Kishorchandra Mandaliya, MBChB, MRCPath, c Jeckoniah O. Ndinya-Achola, MBChB, MSc, b Job J. Bwayo, MBChB, PhD, b and Joan K. Kreiss, MD, MSPH a Seattle, Wash, and Nairobi and Mombasa, Kenya OBJECTIVES: To examine the relationship between use of oral contraceptive pills or depot medroxyproges- terone acetate and sexually transmitted disease acquisition. STUDY DESIGN: Prospective cohort included 948 Kenyan prostitutes. Multivariate Andersen-Gill propor- tional hazards models were constructed, adjusting for sexual behavioral and demographic variables. RESULTS: When compared with women who were using no contraception, users of oral contraceptive pills were at increased risk for acquisition of chlamydia (hazard ratio, 1.8; 95% confidence interval, 1.1-2.9) and vaginal candidiasis (hazard ratio, 1.5; 95% confidence interval, 1.2-1.9) and at decreased risk for bacterial vaginosis (hazard ratio, 0.8; 95% confidence interval, 0.7-1.0). Women using depot medroxyprogesterone ac- etate had significantly increased risk of chlamydia infection (hazard ratio, 1.6; 95% confidence interval, 1.1- 2.4) and significantly decreased risk of bacterial vaginosis (hazard ratio, 0.7; 95% confidence interval, 0.5- 0.8), trichomoniasis (hazard ratio, 0.6; 95% confidence interval, 0.4-1.0), and pelvic inflammatory disease (hazard ratio, 0.4; 95% confidence interval, 0.2-0.7). Consistent condom use was associated with signifi- cantly decreased risk of gonorrhea, chlamydia, genital ulcer disease, bacterial vaginosis, and pelvic inflam- matory disease. CONCLUSIONS: The use of oral or injectable hormonal contraception altered susceptibility to sexually trans- mitted diseases, which may in turn influence transmission of human immunodeficiency virus type 1. Consis- tent condom use was protective with regards to sexually transmitted disease and should be encouraged for the prevention of sexually transmitted disease and human immunodeficiency virus type 1 among women who use hormonal contraception. (Am J Obstet Gynecol 2001;185:380-85) Key words: Hormonal contraception, sexually transmitted diseases, chlamydia, bacterial vaginosis, vaginal candidiasis