We reevaluated conventional criteria for diagnosing coliform infection of the lower urinary tract in symptomatic women by obtaining cultures of the urethra, vagina, midstream urine, and bladder urine. The traditional diagnostic criterion, greater than or equal to 10(5) bacteria per milliliter of midstream urine, identified only 51 per cent of women whose bladder urine contained coliformis. We found the best diagnostic criterion to be greater than or equal to 10(2) bacteria per milliliter (sensitivity, 0.95; specificity, 0.85). Although isolation of less than 10(5) coliforms per milliliter of midstream urine has had a low predictive value of previous studies of asymptomatic women, the predictive value of the criterion of greater than or equal to 10(2) per milliliter was high (0.88) among symptomatic women the prevalence of coliform infection exceeded 50 per cent. In view of these findings, clinicians and microbiologists should alter their approach to the diagnosis and treatment of women with acute symptomatic coliform infection of the lower urinary tract.