Prophylaxis against toxoplasmosis with weekly administration of pyrimethamine/sulfadoxine (Fansidar) was assessed for efficacy and toxicity in bone marrow transplant (BMT) recipients over a 21 month period. Sixty-nine of 90 consecutive seropositive patients were evaluable. Fansidar was administered from the time of established engraftment (median day 40, range days 13-100). Medication was scheduled to be continued until 6 months or longer in cases of continued immunosuppression (median 10 months, range day 72 to 22 months). No proven case of toxoplasmosis occurred in patients receiving prophylaxis. In addition, there were no cases of Pneumocystis carinii. Side-effects included BM suppression requiring cessation (n = 4) or interruption (n = 8) of therapy and rash (n = 1). To evaluate toxicity associated with prolonged therapy, 42 evaluable patients were assessed at 6 months following transplant (or at least 4 months of continuous treatment). Haematological toxicity was minimal and compounded in three patients showing moderate derangement by cytomegalovirus infection and graft-versus-host disease. Fansidar is an effective prophylactic agent against toxoplasmosis in BMT patients.