Nitrofuradantoin was used to treat 36 gravidas with acute urinary tract infections and 93 others with asymptomatic bacteriuria. The drug was found to be safe and effective in clearing bacteriuria in patients infected with sensitive organisms. Treatment of asymptomatic bacteriuria decreased by half the incidence of acute pyelonephritis in the authors' hospital. None of the patients in the asymptomatic group developed symptoms of urinary tract infection. The rate of prematurity in the asymptomatic group was the same as the rate at the hospital.