Six adult male nonrefluxing monkeys were experimentally infected by inoculation of P-fimbriated E. coli into the bladder. Eight control monkeys were inoculated with a non-P-fimbriated E. coli strain. Inoculation with the P-fimbriated E. coli resulted in marked leukocytosis, prolonged bacteriuria and loss of renal function with a 66 per cent incidence of pyelonephritis. Death secondary to bilateral pyelonephritis was seen in 2 monkeys inoculated with P-fimbriated E. coli. Pyelonephritis was not seen in any of the monkeys inoculated with non-P-fimbriated E. coli. The study shows that ascending pyelonephritis can occur in monkeys in the absence of vesicoureteral reflux.