We sent a questionnaire to 22 teams performing allogeneic and autologous bone marrow transplants (BMT) in the UK enquiring about routine use of prophylactic antimicrobials post-transplant, use of CMV-negative blood products and the incidence of major infection acquired more than 3 months post-BMT. Eighteen centres (82%) responded. To prevent Pneumocystis infection 17 centres routinely gave cotrimoxazole at various doses for periods varying between 3 and 12 months (or sometimes longer if chronic GVHD was present) and six centres gave nebulised pentamidine to patients intolerant of cotrimoxazole. Six centres gave penicillin for 1-3 years to allograft patients. Thirteen centres gave only CMV-negative blood products to CMV neg/neg patients, one centre gave CMV immunoglobulin and five centres continued acyclovir to 6 months. During the period 1986-90, 818 autologous and 1007 allogeneic BMT patients were reported, of whom 113 (6.2%) developed severe infections requiring readmission to hospital. The commonest infections were CMV (n = 19), Pneumocystis (n = 12), Pneumococcus (n = 15), Pseudomonas (n = 7) and Aspergillus (n = 8). Some patients with severe infections were not receiving 'appropriate' prophylaxis. Only two of the patients with Pneumocystis were taking cotrimoxazole. We conclude that the duration of continuing prophylaxis against Pneumocystis and pneumococcal infections after BMT needs careful consideration; prophylaxis may be especially important in patients with persisting immune suppression.