Amoxicillin, a synthetic penicillin with a broad antibacterial spectrum, was administed in a daily dose of 1.0 g (in potency) to 47 patients who came to the department of urology (6 patients in whom the antibiotic was used for the prevention of infections included). The results are summarized in the following. 1. The patients with urological infections who were treated with amoxicillin included 20 patients with acute cystitis, 1 with acute urethritis, 18 with chronic prostatitis, 1 with epididymitis, 1 with pyelonephritis and 6 patients in whom the antibiotic was used for the prevention of infections. Amoxicillin was remarkably effective in 27 patients (58.7%), effective in 7 patients (15.2%), but ineffective in 12 patients (26.1%), with the overall effectiveness of 73.9%. The therapy was withdrawn in one patient because of side effects. 2. A patient with acute cystitis showed eruption which was probably associated with allergic reaction to penicillin, and the administration was discontinued. Other side effects worth mentioning were not observed.