The influence of age on tobramycin half-life, volume of distribution, and clearance was examined in 77 patients with infections due to gram-negative bacteria. All patients had normal renal function and hematocrits, were within 20% of their ideal body weight, did not receive penicillin antibiotics concurrently, and had a fever. Twenty-five patients were between 20 and 39 years of age, 23 patients were 40 to 59 years old, and 29 patients were in the age group 60 to 79 years. The mean half-lives were 2.3, 2.2, and 2.4 h, respectively, for the three age groupings. The average clearance and volume of distribution terms were, respectively, 1.34 ml/min per kg and 0.25 liter/kg for the younger group, 1.44 ml/min per kg and 0.26 liter/kg for the middle age group, and 1.25 ml/min per kg and 0.25 liter/kg for the older group. There was no significant difference among the three groups for any of the parameters (P less than 0.05, analysis of variance). Correlation coefficients determined from individual plots of the three pharmacokinetic parameters versus age revealed no correlation between any parameter and age. Forty-five percent of the patients required doses greater than the recommended maximum (5 mg/kg per day) to achieve desired steady-state concentrations. Since tobramycin pharmacokinetics do not change as age increased, doses do not need to be arbitrarily changed in older patients with normal renal function.