We treated 99 female patients suffering from recurrent urinary infections with either 1 gm. methenamine hippurate every 12 hours or 50 mg. nitrofurantoin every 12 hours for intervals of up to 1 year. Monthly microbiological and clinical surveillance was done. Both treatments were effective in reducing the incidence of symptomatic attacks, with nitrofurantoin being more effective. The protective effect of prophylaxis continued after treatment had stopped, suggesting that a permanent beneficial action had been produced. Especially during the first month of treatment methenamine hippurate was tolerated better than nitrofurantoin: 28 per cent discontinued therapy with the latter owing to nausea.