In two successive investigations on nosocomial infections in our hospital, wa have found that asymptomatic bacteriuria is closely related to age (over 50 years) and to treatment with acetylcholine antagonistic activity. We therefore searched for the presence and expression of genes coding for the virulence factors usually present in uropathogenic E. coli in our strains, in strains isolated during asymptomatic bacteriura related to neurologic bladder, and in strains isolated during symptomatic bacteriura. We found that strains from neurologic bladders rarely carried one or two virulence factors while 50% of our strains isolated from asymptomatic bacteriuria carriea at least 3 virulence factors commonly found in strains isolated from symptomatic urinary tract infection. Consequently, it appears important to look for urinary tract infection in patients (over 50 years of age) treated with such drugs, and to look for virulence factors in case of asymptomatic bacteriura. If the stains carry no virulence factors, no antibiotic treatment shoud be instituted but the patients should be invited to drink more water than usual in order to promote elimination of the strains in the urine. Inversely, if the strains carry virulence factors, an adpted antibiotic treatment should be started.