Ten cases of urinary tract infection (6 acute and 3 relapsing) treated with azlocillin are reported. Nine patients had a history of urinary tract catheterization or in-dwelling catheter and 6 had previously undergone surgery. Prior to receiving azlocillin, 6 of the patients had been unsuccessfully treated with other antibiotics. Renal function was altered in 6 cases. Azlocillin was administered intravenously in doses of 5 grams three times or twice a day for periods ranging from 8 days (in 2 cases) to 26 days. The antibiotic was given alone in all but 2 cases where it was associated with amikacin. Excellent clinical and bacteriological results were obtained in 9 of the 10 cases, with only one failure. No local, systemic or biochemical side-effect was recorded. Bacteriological results were most satisfactory. The minimum inhibitory concentrations of azlocillin were lower than those of carbenicillin in all cases. Sensitivity of azlocillin was even good in 3 and intermediate in 2 of the 5 cases where Pseudomonas was indifferent or resistant to carbenicillin. In spite of the small number of cases in this series, these results indicate that azlocillin ranks among antibiotics that are effective in urinary tract infections due to Pseudomonas.