In a retrospective study to evaluate the efficacy of sulfamethoxazole-trimethoprim (SMX-TMP) for the prevention of Pneumocystis carinii pneumonitis, we studied 1760 patients wit hematological malignancies over a twenty-year period (1970-1989). 449 patients received oral SMX-TMP, most of all received 400 mg of SMX and 80 mg of TMP twice per day. None of the patients receiving SMX-TMP developed P carinii pneumonitis, whereas twenty-six (2.0%) of the 1311 patients who did not receive SMX-TMP developed P carinii pneumonitis (p less than 0.01). We found that the SMX-TMP was very effective in the prevention of P carinii pneumonitis in patients with hematological malignancies, and was well tolerated.