During 1991-94 we treated 51 patients with acute myeloid leukaemias and 3 patients with a myelodysplastic syndrome of refractory anaemia with excess of blasts in transformation. The patients received trimethoprim-sulphamethoxazole (TMP-SMX) 1,920 mg daily as a prophylaxis of Pneumocystis carinii infections and selective decontamination of gastrointestinal tract. The majority of patients received TMP-SMX in their first course of chemotherapy with daunorubicin and cytosine arabinoside. Only one of the 18 patients without TMP-SMX prophylaxis during the first course of chemotherapy developed Pneumocystis carinii pneumonia. That pneumonia was successfully treated by intravenous administration of TMP-SMX 1920 mg four times a day. No other Pneumocystis carinii infection was encountered in all other patients during their clinical follow up or in autopsy material of expired patients. TMP-SMX prophylaxis had to be interrupted in 11 patients due to their suspicious allergic skin reactions, however, TMP-SMX was readministered in all without any skin changes attributable to TMP-SMX during next cycles of chemotherapy. TMP-SMX in a given daily dose of 1,920 mg seems to be a successful prophylaxis of Pneumocystis carinii infections in patients with malignant diseases of hematopoiesis.