134 Gambian children under 5 years of age with severe pneumonia (as defined by the World Health Organisation classification of acute respiratory infections) were given either oral co-trimoxazole for 5 days, or a single intramuscular dose of fortified procaine penicillin and 5 days of oral ampicillin. At 2 weeks, there was no significant difference in outcome between the two groups. Co-trimoxazole is much less expensive than ampicillin or procaine penicillin, requires only twice-daily administration, and can be given by health-care staff with little training. The results support the use of co-trimoxazole as the antibiotic of first choice in outpatient management of young children with pneumonia in developing countries.