To assess the urovirulence characteristics of Escherichia coli strains causing acute prostatitis, urinary isolates from men with acute prostatitis (n=107) and from women with acute uncomplicated pyelonephritis (n=76) were examined for the prevalence of sfa, foc, and 3 papG allele genotypes and phenotypes and for the production of alpha-hemolysin and cytotoxic necrotizing factor 1. The papG allele III and foc gene were found more frequently and the papG allele II less frequently among prostatitis than from pyelonephritis isolates. A higher proportion of hly+ cnf1+ genotype in prostatitis strains (64% vs. 36%) was particularly striking. Both prostatitis and pyelonephritis strains expressed virulence factors similarly except for a higher proportion of nonhemolytic prostatitis isolates. Although the pathogenetic mechanisms of urinary tract infections in men and women may differ, virulence factors such as adhesins and cytotoxins may have important roles in the pathogenesis of acute prostatitis.