The efficacy of the dosing regimen of tobramycin was investigated in 28 patients with cystic fibrosis who had an acute exacerbation of chronic pulmonary infection with Pseudomonas aeruginosa. The initial dose of tobramycin was 3.3 mg/kg of body weight three times daily (ie, 10 mg/kg/day). A highly significant relationship was found between the serum concentration of tobramycin before the dose and the change in the forced expiratory volume in one second (FEV1), both measured on the tenth day of treatment (rs = 0.75; p less than 0.001). In nine of the 16 patients who had a six-hour serum concentration of 1 mg/L or less on the tenth day of treatment, the eight-hour dosing interval of tobramycin was shortened to achieve a serum concentration of tobramycin of about 1 mg/L before the dose. In the other seven patients, the dosage of tobramycin was not changed. On the 20th day, seven of the nine patients in whom the dosing interval was shortened exhibited an increase in FEV1 of 20 percent or more. Such an increase was observed only in one of the seven patients in whom the dosing interval was not reduced (p less than 0.05). We conclude that individualizing the dosage of tobramycin in patients with cystic fibrosis results in a better clinical outcome.