The prevalence of hemolysin, type 1 fimbriae, P fimbriae, cytotoxic necrotizing factor-1 (CNF-1), aerobactin, and autotransporter toxin (sat) was analyzed by polymerase chain reaction and phenotypic assays of 42 epidemiologically unrelated Escherichia coli strains causing acute pyelonephritis in women (21 nalidixic acid-susceptible and 21 nalidixic acid-resistant strains) and 58 E. coli strains causing cystitis in women (29 nalidixic acid-susceptible and 29 nalidixic acid-resistant strains). Hemolysin and CNF-1 were less prevalent (P < .05) in nalidixic acid-resistant than in nalidixic acid-susceptible E. coli strains from patients with either pyelonephritis (14.3% vs. 52.4%) or cystitis (0% vs. 31.0%). Among E. coli strains causing cystitis, type 1 fimbriae expression was less prevalent (P < .05) in the nalidixic acid-resistant group (55.2%) than in the nalidixic acid-susceptible group (86.2%). None of the nalidixic acid-resistant and 20.7% of the nalidixic acid-susceptible strains causing cystitis showed the proteolytic toxin Sat (P < .05). These results suggest that resistance to quinolones may be associated with a decrease in the presence or the expression of some virulence factors in uropathogenic E. coli.