Pivmecillinam (PMPC), semisynthetic penicillin for oral use, was studied clinically and following results were obtained. 1) Twenty-eight patients with acute simple cystitis treated with the drug 200 mg/day for 4 days, clinical results were excellent in 23 cases and moderate in 5 cases. Six patients with chronic simple cystitis treated with the drug 400 mg/day for 7 days, clinical results were excellent in 4 cases, moderate in 1 case and poor in 1 case. Eight patients with complicated urinary tract infections treated with the drug 400 mg/day for 7 days, clinical results were excellent in 2 cases, moderate in 2 cases and poor in 4 cases. Overall effectiveness amounted to 88.1%. 2) Pivmecillinam was clinically effective in patients infected by Gram-negative bacteria except Pseudomonas aeruginosa. 3) No side effects were observed in all cases. The effectiveness of PMPC for acute simple cystitis was compared with that of talampicillin (TAPC), ampicillin (ABPC), amoxicillin (AMPC) and pivampicillin (PVPC) reported previously and PMPC was assessed as the most useful agent in these. Then, pivmecillinam should be chosen firstly as a chemotherapeutic agent for acute simple cystitis.