Reported are the results of a randomized trial of sulfamethoxazole + trimethoprim versus procaine penicillin for the outpatient treatment of pneumonia in 614 children aged 3 months to 12 years at primary health care clinics in Chitungwiza, a large town near Harare, Zimbabwe. Diagnosis and treatment were carried out by nurses, without medical supervision. The presence of lower respiratory tract infection that required antibiotics was diagnosed on the basis of a recent history of a cough and the presence of a respiratory rate of greater than 50 per minute. Patients were followed up by a research nurse with minimal drop-out losses. Referred children were examined and assessed by a doctor at the Chitungwiza General Hospital. Of the study children, 65 (11%) were referred to hospital, but only 8 (1.3%) had pneumonia that required a change in the treatment (5 in the sulfamethoxazole + trimethoprim group and 3 in the procaine penicillin group). There were no significant differences in outcome between the two treatment groups. One child, who had evidence of infection with human immunodeficiency virus (HIV), died. Sulfamethoxazole + trimethoprim and procaine penicillin were highly and equally effective for the outpatient treatment of children who had been clinically diagnosed to have pneumonia by primary health care workers.