- In order to monitor changes in the frequency and antimicrobial resistance of urinary pathogens over several years, urinary cultures received from outpatient clinics and from a hospital during a period of one month each in 1991 and 1995 were analyzed at a clinical microbiology laboratory. In 1991 and 1995, 1366 and 1534 significant monomicrobic cultures respectively were reviewed. The frequency of Escherichia coli dropped significantly in the outpatient clinics from 70.5% to 61.2% (p < 0.0001). The frequency of Proteus mirabilis, Morganella morganii, Pseudomonas aeruginosa and other gram-negative bacteria also decreased, but the frequency of Klebsiella spp. and Enterobacter spp. increased from 2.6% to 5.8% (p < 0.0001). In the hospital, the frequency of Enterobacter spp. (p < 0.04), Escherichia coli and Morganella morganii declined from 1991 to 1995, whereas the frequency of Pseudomonas aeruginosa (p = 0.001), Acinetobacter spp. (p < 0.05), Klebsiella spp., Proteus mirabilis and other gram-negative rods increased considerably. The frequency of gram-positive aerobic bacteria rose markedly in outpatient specimens from 6.1% to 13.5% (p < 0.0001), while a decline from 14.4% to 9.3% was noted in hospital specimens (p < 0.02). A significant rise in the resistance of Escherichia coli to gentamicin and ciprofloxacin (p < 0.0001) was detected in outpatient isolates. In the hospital, gram-negative urinary pathogens demonstrated increased resistance to ampicillin (p = 0.042), cefuroxime (p = 0.005), gentamicin (p = 0.002) and ciprofloxacin (p < 0.0001) during the study period. The changing etiology of urinary tract infections and the increasing resistance of organisms indicate that periodic monitoring and possibly also modification of empirical therapy are required.