Tobramycin pharmacokinetics is usually described by a one-compartment model, but this model fails to account for both the incomplete urinary recovery and prolonged post-treatment persistence noted with this drug. We examined the multiple-dose behavior of tobramycin in 35 treated patients with stable renal function, using peak and trough serum concentrations, urine recovery, and postmortem tissue analysis. Serum concentrations rose slowly throughout treatment and declined in two phases after the drug was stopped. The first-phase half-life correlated well with renal function, but the second averaged 146 h and was poorly related to creatinine clearance. A two-compartment model was used to describe the biphasic decline in serum concentrations and to calculate the amount of drug in the tissue compartment at all times during and after treatment. Predicted tissue amounts rose continually throughout treatment in all study patients. In 5 patients, the total amount of tobramycin in the body after the final dose was recovered in the urine, but urine had to be collected for 10 to 20 days to achieve complete recovery of the drug. In four patients, the predicted tissue amount was recovered from postmortem tissues. Regardless of the dose, tobramycin accumulated in the tissues of all patients receiving this antibiotic. The two-compartment pharmacokinetic model explains both the rising peak and trough concentrations during treatment and the detection of the drug in serum and urine long after the last dose.