Outcomes of searches for unrelated bone marrow donors were analysed to identify factors influencing the probability of identifying an HLA-matched donor and proceeding to an unrelated donor bone marrow transplant (UD-BMT). Between March 1989 and January 1991, 649 unrelated marrow donor searches were entered into the study. Searches were referred from transplant centres in Europe and North America to the Anthony Nolan Research Centre panel and/or the British Bone Marrow and Platelet Donor Panel, two predominantly Caucasian donor registries. Patient immunogenetic and clinical characteristics were documented on study search request forms and search outcomes were monitored at the donor registry. Data were analysed using proportional hazards regression. Major factors predicting favourable search outcomes were patient common Caucasian HLA phenotype and Caucasian ethnic group. Probability of search failure was increased by advanced disease, rare Caucasian HLA phenotype and referral to the smaller registry. Search failure frequently occurred at the mixed lymphocyte reaction stage. This study illustrates the relative roles of clinical, immunogenetic and registry factors in determining the outcome of searches. The information may be used to devise strategies to locate HLA-matched donors for a higher proportion of patients for whom UD-BMT is the preferred treatment.