Amikacin's pharmacokinetics and dosage requirements were studied in 98 patients receiving treatment for gram-negative infections. A wide interpatient variation in the kinetic parameters of the drug occurred in all patients and in patients who had normal serum creatinine levels or normal creatinine clearance. The half-life ranged from 0.7 to 14.4 h in 74 patients who had normal serum creatinine levels and from 0.7 to 7.2 h in 37 patients who had normal creatinine clearance. The necessary daily dose to obtain therapeutic serum concentrations ranged from 1.25 to 57 mg/kg in patients with normal serum creatinine levels and from 10 to 57 mg/kg in patients with normal creatinine clearance. In four patients (4%), a significant change in baseline serum creatinine level (>0.5 mg/dl) occurred during or after treatment, which may have been amikacin-associated toxicity. Overt ototoxicity occurred in one patient. The method of individualizing dosage regimens provided a clinically useful means of rapidly attaining therapeutic peak and trough senum concentrations.