Pivmecillinam (FL 1039) is the pivaloyloxymethyl ester of mecillinam (FL 1060) which has considerable in vitro activity against Enterobacteriaceae. 38 hospital inpatients who had proven urinary tract infections were treated with 400 mg pivmecillinam four times daily for 5–7 days. The MIC of mecillinam to the infecting organisms was determined as were the serum and urinary concentrations of the antibiotic. The patients were followed up for 4–6 weeks after the end of treatment. Three patients were lost to follow-up. Of the 35 patients who were adequately followed up, 29 (83%) were classified as cured and there were 6 failures. Reported side effects were of a minor nature.