To study the effects of continous low doses of antibacterial agents after eradication of bacteriuria in patients with recurrent urinary tract infection, 31 patients with documented recurrent urinary tract infection were allocated alternately to treatment with either co-trimoxazole (400 mg of suphamethoxazole and 80 mg of trimethoprim each night) or methenamine hippurate (1 g each night). The majority of patients (79%) had underlying radiological abnormalities of the renal tract, but normal renal function (the mean serum creatinine level was 1.05 mg per 100 ml). During the study the incidence of bacteriuria and pyuria was significantly lower in the co-trimoxazole-treated group. Patients receiving co-trimoxazole also had fewer acute clinical episodes of urinary tract infection than patients receiving methenamine hippurate. There were no significant side effects from either drug. Two patients with frequent recurrences of infection developed renal calculi. No change in creatinine clearance or maximum urinary concentrating ability was observed over a follow-up period of four to 30 months (mean 10.4 months). Co-trimoxazole is an effective agent in the prophylaxis of urinary tract infection in this highly susceptible group of patients, and in the doses used was superior to methenamine hippurate.