Substantial interpatient variations were demonstrated in the daily doses required to obtain therapeutic gentamicin sulfate serum concentrations in 417 elderly patients. Dosages ranged from 0.3 to 22.0 mg/kg/day in patients with a normal serum creatinine level. Twenty-five percent of these patients required daily doses higher than the standard regimen of 5 mg/kg/day, and 33% required less than 3 mg/kg/day. The drug half-lives in these patients ranged from 0.3 to 32.7 hours, compared with previous reports of 2.5 to four hours. The distribution volumes of these patients ranged from 0.07 to 0.53 L/kg, compared with reported values of 0.20 to 0.25 L/kg. These wide variations in kinetic variables in elderly patients and the need to obtain narrow ranges in serum concentrations required measuring serum concentrations and individually calculating each patient's dosage requirement early in the treatment course. Doing this consistently produced optimal peak and trough serum levels. Ototoxicity did not occur in any of the patients, and nephrotoxicity may have been drug related in 2% of the elderly patients.