Abstract The clinical effects of treatment with a trimethoprim (TMP)-sulphamethoxazole (SM) combination were studied in 40 patients with urinary tract infection. The indication for treatment was symptoms of urinary tract infection with > 10(5) colonies per ml of urine. The dosage was 160 mg of TMP and 800 mg of SM administered orally twice daily for 10 days. The bacteriological in vitro examination revealed synergic effect of the two agents on 30 out of 55 bacterial strains (55%). Out of 32 strains which were resistant or slightly sensitive to SM, synergy was revealed in 14 instances (44%). Development of resistance to TMP was seen in one case. In 20 patients with non-complicated urinary tract infections, sterile urine was found 3 and 14 days after termination of treatment in 100% and 89%, respectively. In one case there was recurrence, and in one case reinfection occurred. In 20 patients with complicated urinary tract infections, sterile urine was found 3 and 14 days after termination of treatment in 80% and 55%, respectively. In 4 cases infection recurred, and in 5 cases the treatment had no effect. Side effects were seen in 3 patients; one case of exanthem, one of nausea, and one of leucopenia. In one case treatment had to be withdrawn because of exanthem, in another because of nausea.