Effective prevention of recurrent urinary tract infections (UTIs) and possible acute pyelonephritis is of major importance during pregnancy. During 39 pregnancies, 33 women with a history of recurrent UTIs (and, in some instances, pyelonephritis) received postcoital prophylaxis consisting of a single oral dose of either cephalexin (250 mg) or nitrofurantoin macrocrystals (50 mg). While 130 UTIs occurred during a mean observation period of 7 months before prophylaxis, only a single UTI occurred during pregnancy after prophylaxis; this difference was highly significant. Thus the use of one of these effective regimens is strongly recommended for any pregnant woman with a history of recurrent UTIs. The treatment's effectiveness can be explained by two features of the two antibacterial agents involved: both reach high bactericidal concentrations in the urinary tract and induce no (or minimal) resistance in the introital gram-negative bacterial flora.