In a double blind, randomized study, sulfamethoxazole was compared alone and in combination with trimethoprim as commonly used in therapeutic regimes for the treatment of uncomplicated acute urinary tract infections in out-patients. The cure of sulfamethoxazole alone was 92.2%, and for sulfamethoxazole plus trimethoprim 97.6%. The rate of side-effects for the former was 5%, for the latter 21.8%. If the failure rate plus the rate of occurrence of rash, which necessitated discontinuing the drug, are combined, it appears that 8.8% of the patients were at a disadvantage receiving sulfamethoxazole compared to 9.7% for the combination of sulfamethoxazole plus trimethoprim. When considering the cure rate and rate of side-effects together, therefore, the position of sulfamethoxazole alone as a suitable drug in this type of infection is defended.