The pharmacokinetics of tobramycin in patients with cystic fibrosis before and after lung transplantation were evaluated. Twenty-nine lung transplant recipients with cystic fibrosis who received at least one course of tobramycin pre- and posttransplantation were included in this study. Pharmacokinetic parameters (clearance, volume of distribution, elimination rate and half-life) were calculated using a one-compartment Bayesian method. Comparisons were made both between and within pre- and posttransplant periods for patients receiving multiple courses. Significant differences were noted. Clearance was decreased 40%, volume of distribution increased 20%, elimination rate increased 52%, and half-life increased 141%, respectively, posttransplant as compared to pretransplant. There were no differences within each time period between each tobramycin course. The results indicate that tobramycin pharmacokinetics are significantly altered in patients with cystic fibrosis after lung transplantation. Patients with cystic fibrosis require early and close monitoring of tobramycin after lung transplantation.