This study was undertaken to examine the impact of endogenous digoxin-like immunoreactive substance (DLIS) on the therapeutic drug monitoring of digoxin in elderly patients. Serum digoxin concentrations were detected by TDx digoxin assay in 220 patients 65 years of age and older who were not administered cardiac glycosides, spironolactone, or canrenone. The influence of clinical factors and laboratory parameters on the presence of DLIS was also evaluated. The concentration of DLIS averaged .07 ng/ml (range, 0-0.74 ng/ml), and in 25 subjects DLIS was observed over the detection limit of the assay. None of the parameters demonstrated a significant contribution to the presence of DLIS except one: continuous intravenous drip infusion therapy with a sodium-containing solution (DIV). The frequency of DLIS was significantly higher in the elderly subjects than in 121 control subjects younger than 35 years of age (p = 0.01). However, the difference became insignificant when subjects administered DIV were excluded from the analysis (p = 0.15). It was suggested that DIV may be clinically important in indicating the presence of DLIS.