The efficacy of a single dose trimethoprim in the treatment of the dysuria/pyuria syndrome in women was prospectively studied in an open, controlled and randomized trail. Of 90 women aged 18-60 years 68 had and 22 had not significant bacteriuria when the urine was cultured. The 42 women in the single-dose group received trimethoprim 400 mg once while the 48 in the multi-dose group received trimethoprim 200 mg X 2 for five days. One week after ended treatment 83% were cured in the single-dose group, and 85% in the multi-dose group. One relapse and six reinfections occurred in the single-dose group, no relapse and seven reinfections in the multi-dose group. Two patients in the single-dose group and eight in the multi-dose group reported adverse effects. A single dose of trimethoprim seems to be safe and effective in the treatment of acute dysuria/pyuria syndrome in women.