There is evidence that routine catheterization of the urethra at the time of delivery has contributed to the frequency of postpartum bacteriuria. The possibility of preventing bacteriuria under these conditions was studied in 415 healthy women who had normal spontaneous deliveries preceded by routine catheterization. Of these, a test group of 217 received antimicrobial (sulfamethoxypyridazine) therapy whereas a contratest group of 198 was treated with a placebo. Two patients in the test group and 17 in the contratest group developed symptomatic urinary-tract infection during the study. The effectiveness of the prophylaxis was clearly demonstrated. Bacteriuria appearinwhere-r delivery did not persist, whereas patients with evidence of infection at the time of delivery tended to have bacteria in the urine at the visit 6 weeks post partum.