We have studied three circumstances that have been reported to make interpretation of the serum digoxin concentration difficult in patients with renal failure: increased biotransformation; endogenous digitalis-like factors (DLF); and sudden, unexpected increases in serum digoxin values, even after the discontinuation of digoxin. Biotransformation, as estimated by the percent true digoxin in serum, was comparable in patients with renal failure who were dependent on dialysis and in control subjects (76% vs. 73%). Certain commercial immunoassays did not, or rarely, gave values for DLF of clinical significance (greater than 0.2 ng/ml digoxin equivalents) in patients with a wide range of renal dysfunction who were not receiving digoxin. With a sensitive method, values for DLF did not exceed 0.23 ng/ml in 22 dialysis patients dependent on dialysis, but were significantly increased in comparison with values in control subjects. The case histories of two patients with renal failure, acute illness, and sudden unexpected marked increases in serum digoxin concentrations are presented and possible explanations are discussed.