Thirty-nine subjects with active non-obstructive urinary tract infection due to common pathogens were treated in a randomized fashion either with sulfamethoxazole alone (22 patients) or a combination of trimethoprim-sulfamethoxazole (17 patients) during four consecutive weeks in the course of a controlled study. The combination trimethoprim-sulfamethoxazole appeared more effective than sulfamethoxazole alone in eradicating infection. Complete disappearance of urinary tract infection was obtained in 82% of subjects treated with the combination versus 59% in those treated with sulfamethoxazole alone. Recurrence of infection averaged 43% in the group treated with the combination and 54% in those treated with sulfamethoxazole alone. The mean time interval between successful response and recurrence of infection was 17 weeks for the combination and 20 weeks for sulfamethoxazole alone. When the comparison of the results between sulfamethoxazole alone and the combination trimethoprim-sulfamethoxazole is restricted to the subjects with E. coli infection (69% of all cases), no important difference between the two groups can be demonstrated. A skin rash which sometimes disappeared spontaneously despite continued medication was the only side effect noted with the trimethoprim-sulfamethoxazole combination.