A program for gentamicin and tobramycin dosage calculations using a pocket-size programmable calculator was developed and evaluated. A program based on the first-order one-compartment pharmacokinetics model for aminoglycoside dosage calculations was developed for the Sharp EL-5813 pocket-size calculator, which can store 30 programming steps and has six constant memories. (The program also can be used in other calculators with similar capabilities.) The programming steps and operational procedures for calculating loading and maintenance doses, steady-state peak and trough concentrations, the patient's elimination constant, creatinine clearance, and volume of distribution are presented. The program was evaluated by comparing predicted and observed steady-state peak and trough concentration for 24 patients receiving gentamicin therapy. There was a significant relationship between predicted and observed peak and trough plasma concentrations. The mean time required to program the calculator was less than 30 seconds, and the mean time required for each dosage and plasma-concentration calculation was approximately 2.5 minutes. The program provides a simple method to make aminoglycoside dosage recommendations and predictions of peak and trough plasma concentrations with acceptable accuracy.