The ureidopenicillin piperacillin was given to 20 patients with urinary tract infections. The infecting microbes all had a minimum inhibitory concentration below 0.6 microgram/ml. One strain of Staphylococcus aureus produced beta-lactamase. All patients were freed of their microbes at the end of 7 days therapy. 1--2 weeks after the end of therapy, 1 relapsed and 3 had superinfections. Among the 16 who were without bacteria or leucocyturia after 1--2 weeks, 2 exhibited a superinfection 4--6 weeks after the end of therapy.