Eighty patients with urinary tract infections were randomly divided into two equal groups; one group received tobramycin, and the other received gentamicin. The daily dose of antibiotic for each group was 3 mg/kg. Mean levels in serum were 4.5 and 4.7 mug/ml, respectively, 1 hr after an intramuscular injection, and the mean duration of therapy was 6.5 days. Mean ages and sex ratios were similar in the two groups. Essentially equal numbers of patients with pyelonephritis and cystitis were assigned randomly to each group. Eradication of infection was similar in patients in the two treatment groups, and there was not a significant number of relapses or reinfections in either group. Renal function was abnormal in seven patients treated with gentamicin and in two patients treated with tobramycin. One patient in the gentamicin group developed a skin eruption. Our data suggest that tobramycin is as effective as gentamicin in the treatment of acute urinary tract infection and may be less nephrotoxic at similar doses.