The efficacy and adverse effects of prophylactic administration of Sulfamethoxazole-Trimethoprim (ST) for Pneumocystis carinii Pneumonia (PCP) were assessed in patients with connective tissue diseases (CTD). Eighty-four patients who were receiving more than 40 mg/day of prednisolone were entried in the present study. Patients with at least one of the two PCP risk factors (interstitial pulmonary fibrosis and lymphopenia), were administered either one (11 patients) or two (26 patients) ST tablets/day. The remaining 47 patients who did not receive ST served as the controls. Although PCP was detected in 4.3% of the patients in the no-ST group, none of the patients who received ST developed PCP. Five of these 26 patients who received two tablets of ST/day, experienced adverse reactions. However, no adverse reactions were detected in the patients who received one tablet of ST/day (p < 0.05). Abnormal laboratory data were obtained for 10 (38.5%) of the patients who received two tablets of ST/day and for 4 (36.4%) of the 11 patients who received one tablet of ST/day. The results of the present study suggest that the prophylactic administration of one tablets of ST in patients with CTD that have at least one of the two PCP risk factors is effective in preventing PCP.