The usefulness of pharmacokinetic predictions of serum digoxin concentrations was evaluated in a group of 19 elderly residents of a nursing home. A statistically significant correlation between the calculated and measured digoxin level was found with a 95% confidence limit of 0.55 ng/mL (0.7 nmol/L). Although the use of liquid-filled digoxin capsules and the utilization of the equations described in this article resulted in improved predictability of steady-state serum digoxin concentrations, the calculated serum digoxin level significantly underestimated the measured value in 33% of the patients who were not receiving any medications known to increase serum digoxin concentration. Further studies are needed to improve the reliability of pharmacokinetic models in estimating serum digoxin concentrations in the elderly.