Postoperative urinary tract infections (UTIs) in renal transplant patients were studied before and after introduction of a protocol requiring single-dose perioperative antibiotics and earlier catheter removal. The overall incidence of UTIs was reduced from 55.4% to 26%. The most dramatic reduction was in nondiabetic males, from 56% to 8.2%. There was a small but statistically insignificant reduction in infection rates in females. There was no change in the rate of infection in diabetics. The incidences of noncoliform and mixed infections, in the group as a whole, were dramatically reduced from 42.9% to 12%, but Escherichia coli infections were totally unaffected. This protocol exposed the special susceptibility of women and diabetics to posttransplant UTIs and the different pathogenesis of E coli versus noncoliform infections. These features need more study.