Fifty-three patients with cystic fibrosis were treated with large doses of tobramycin sulphate, 10 mg/kg/24 h. Patients received an average of 3.2 courses, each lasting 10 to 14 days. The patients were examined with audiometric and vestibular tests, and in one patient, a 10-year-old girl, we found a transient high tone loss at 8000 Hz bilaterally that might be attributed to the tobramycin treatment. All patients had normal kidney function. High dose tobramycin treatment in children with normal kidney function gives minimal clinical risks of adverse ototoxic effects, even after repeated courses of treatment.