1) Forty (40) patients with comparatively simply urinary tract infections were orally administered 1 g/day of sustained release cephalexin (S-6436) in two divided doses (after breakfast and dinner) for 7 approximately 14 days, and 20 patients complicated with chronic urinary tract infections or epididymitis were orally given 2 g/day of the drug in two divided doses (after breakfast and dinner) for 7 approximately 28 days. Thirty six (90%) patients of the 40 receiving 1 g/day and seventeen (85%) of the 20 with 2 g/day satisfactorily responded to the drug. 2) Disappearance of organisms was observed in 43 patients (75.4%) of the 57 with urinary tract infections on whom bacteriological examination was conducted. Three (7.0%) of the 43, however, recurred within 7 days after the treatment. In six patients (10.5%) of the 57, the organisms were replaced. 3) Side effects such as gastrointestinal disturbance, allergic reactions, abnormality of blood picture, and impairments of renal and hepatic functions were examined and no such side effects were found except uriticaria in whole body was observed in only one patient with 1 g/day, which disappeared after discontinuation of the administration of the drug.