Trimethoprim was used alone to treat urinary tract infections in 20 women who were unable to tolerate sulfonamides. Of ten acute symptomatic urinary tract infections, four were cured, three were not, and three cases could not be evaluated. Two other women received trimethoprim for suppression of infection complicating stag-horn calculi. The conditions of both patients improved clinically but the urine remained infected. Eight women treated prophylactically with low-dose trimethoprim for recurrent urinary tract infection accumulated a total of 16 patient-years of prophylaxis. During treatment, the incidence of infection was 0.56 per patient-year compared with 4.25 infections in the year preceding study. Adverse reactions occurred in eight of 20 patients and administration of the drug had to be stopped in five cases. Trimethoprim alone is effective for the treatment and prophylaxis of urinary tract infections, but may cause a high incidence of adverse reactions in patients known to be sensitive to sulfonamides.