Serum digoxin levels were examined in 55 patients with varying degrees of renal function impairment who were receiving chronic oral digoxin therapy. Three methods of predicting digoxin serum levels were investigated. Each method may be applied using serum creatinine values and does not require urinary data. Correlations between calculated and actual digoxin levels in combined male and female patients were improved when changes in digoxin distribution volumes in renal impairment were considered. Correlations between calculated and actual digoxin levels were poor in male patients but were again improved by incorporating changes in drug distribution volume. Correlations obtained in female patients were superior to those obtained in male patients and appeared to be independent of the method of calculation employed.