Nalidixic acid (NA) was administered to female patients with acute simple cystitis at a dose of 2g daily for 3 days to determine its efficacy. Recurrence rate and factors causing recurrence were also discussed based on the results of a follow-up survey.Upon completion of a 3-day treatment with NA at a dose of 2g daily, efficacy of the drug was assessed according to the Criteria for Drug Evaluation by UTI Study Group. 430 out of 808 patients collected from 103 institutions were subjected to the evaluation. The remaining cases were excluded because of various reasons. 258 (60%) out of the 430 cases showed “Excellent” response to NA and “Good” in 110 patients (25.6%). The overall efficacy rate was 85.6%.Then the follow-up treatment for 7 days was done in 368 patients of “Excellent” or “Good” and reexamined. Inter-group comparison was made, where Group A received NA 1g for 7 days and a low dose of antiinflammatory agent was given to Group B for a week following the initial treatment. No treatment was done in Group C following the initial treatment. The recurrence was less frequent in Group A, while higher incidence of recurrence was observed in Groups B and C. It was found that the recurrence rate in those patients showing “Good” response to the initial 3-day treatment with NA was higher than that of the “Excellent” Group. It is to note that relatively high incidence of recurrence was observed in those patients with persisting bacteria in urine, even if its bacterial count was less than 104/ml.