In a prospective, randomized, double-blind trial, a regimen of 250 mg of moxalactam every 12 hours was compared with 1.0 mg/kg of tobramycin every eight hours in the treatment of urinary tract infections. One hundred and eleven patients were entered into the study; results in 63 (18 men and 45 women) were evaluable for both efficacy and toxicity. Thirty evaluable patients received moxalactam, and 33 received tobramycin. The mean duration of therapy in each group was seven days. There were six treatment failures in the moxalactam group and 10 failures in the tobramycin group (p > 0.4). Nephrotoxicity, defined as an increase in serum creatinine levels to 0.5 mg/dl or more, did not occur in either group. Thirteen patients in the moxalactam group and one in the tobramycin group had enterococci isolated from a urine culture specimen during or after therapy. It is concluded that use of the moxalactam regimen Is as effective and safe as use of the tobramycin regimen in the treatment of urinary tract infections. The clinical significance of the enterococcal isolates associated with moxalactam therapy is yet to be determined.