British Journal of Haematology , 2001, 112, 219±227 Factors influencing haematological recovery after allogeneic haemopoietic stem cell transplants: graft-versus-host disease, donor type, cytomegalovirus infections and cell dose Alida Dominietto, 1 Anna Maria Raiola, 2 Maria Teresa van Lint, 2 Teresa Lamparelli, 2 Francesca Gualandi, 2 Giovanni Berisso, 2 Stefania Bregante, 2 Francesco Frassoni, 2 Lucia Casarino, 3 Simonetta Verdiani 2 and Andrea Bacigalupo 2 1 Divisione Universitaria di Ematologia, Azienda Ospedaliera S. Giovanni Battista, Torino, 2 Dipartimento di Ematologia Ospedale San Martino, and 3 Cattedra Medicina Legale, Universita 0 di Genova, Genova, Italy Received 1 May 2000; accepted for publication 28 July 2000 Summary. Platelet recovery after allogeneic haemopoietic stem cell transplant (HSCT) and predictive factors were analysed in 342 patients with haematological malignancies. All patients were prepared with cyclophosphamide plus total body irradiation, and received an unmanipulated HSCT from an HLA-identical sibling (n  270), a matched unrelated donor (n  67) or an identical twin (n  5). The source of stem cells was peripheral blood (n  15) or bone marrow (n  327). Graft-vs.-host disease (GvHD) prophylaxis consisted of cyclosporin A with or without methotrexate. The proportion of patients with , 50  10 9 /l platelets on d 150, d 1100, d 1200 and d 1365 after HSCT was 26%, 27%, 14% and 11% respectively. Thrombo- cytopenia was independent of the degree of complete donor chimaerism. Four variables were predictive of platelet recovery: donor type, acute GvHD, cytomegalovirus (CMV) infection and number of cells infused at transplant. Recipients of an unrelated graft had lower platelet counts (49  10 9 /l) on d 150 than identical sibling grafts (108  10 9 /l) (P , 0´001) and twin grafts (149  10 9 /l) (P , 0´001). Patients with GvHD grades 0, I, II, III and IV had significantly different platelet counts on d 150 (153  10 9 /l, 102  10 9 /l, 85  10 9 /l, 32  10 9 /l and 22  10 9 /l; P , 0´001) and thereafter. Thrombocytopenia was more frequent in patients with high-level CMV anti- genaemia (. four positive cells/2  10 5 ) (P , 0´0001) and in patients who received a low cell dose at transplant (# 4´1  10 8 /kg) (P  0´009). Platelet counts predicted transplant-related mortality (TRM) and were higher at all time intervals in patients surviving the transplant. Patients with grade II GvHD and . 50  10 9 /l platelets had a lower TRM than patients with grade II GvHD and # 50  10 9 /l platelets (14% vs. 40%, P , 0´0001). In conclusion, (i) a significant proportion of allogeneic HSCT recipients are thrombocytopenic long-term, irrespec- tive of complete donor chimaerism, (ii) thrombocytopenia identifies patients at greater risk of lethal complications, and (iii) platelet recovery is influenced by GvHD, donor type, CMV infections and cell dose, not by stem cell source or other patient±disease-related variables. Keywords: allogeneic bone marrow transplantation, haematopoietic recovery, graft-versus-host disease. Allogeneic haemopoietic stem cell transplant (HSCT) is an established form of therapy for several haematological disorders (Hansen et al, 1998; Socie  et al, 1999). Patients are prepared with high-dose chemo-radiotherapy, followed by intravenous infusion of haemopoietic stem cells. Shortly after infusion, stem cells leave the circulation, home to extravascular spaces (Lanzkron et al, 1999) and re-populate the recipient's bone marrow. Different molecular mechan- isms involving adhesion molecules, chemokines and para- crine cytokines have been shown to regulate transendothelial migration of haemopoietic progenitor cells (Mohle et al, 1999). When myeloid and lymphoid cells are of donor origin, this is taken as proof of engraftment and referred to as complete donor chimaerism (McSweeney & Storb, 1999). Engraftment of stem cells depends on several factors such as (i) the intensity of the preparative regimen (Storb et al, 1989; Terenzi et al, 1990), (ii) the grafted cell dose (Sierra et al, 1997; Reisner & q 2001 Blackwell Science Ltd 219 Correspondence: A. Dominietto, M.D., Dipartimento di Ematologia PAD 5/2, Ospedale San Martino, 16132 Genova, Italy. E-mail: apbacigalupo@smartino.ge.it