A total of 32 adult patients of both sexes with chronic recurring urinary tract infections were treated with tobramycin in a daily dose of 1.2 mg/kg of body weight. They were randomly allocated to receive the drug at 8 or 12 hour intervals by intramuscular injection. The overall immediate disappearance of bacteriuria after therapy was 78.1 per cent; 15 days later, the cure rate dropped to 60 per cent. There was no difference between the two schedules tested. Tolerance to tobramycin was excellent. No reaction was recorded at the injection site; audiometry (performed in nine patients) and laboratory tests done prior to and after therapy did not reveal any hematologic, hepatic and/or renal toxicity, with one exception. In a patient with chronic renal failure (a patient with a solitary kidney infected with a pseudomonas sp.) there was a consistent elevation of blood urea in two separate courses of tobramycin, returning to normal within two weeks after stopping the therapy.