The objective of this study was to assess the effect of endogenous digoxin-like substances on the interpretation of excessive concentrations of digoxin in children. After the development of a high-pressure liquid chromatography (HPLC) method for digoxin in our laboratories, we analyzed sera of children in whom the fluorescence polarization immunoassay identified potentially toxic concentrations of the glycoside (greater than 3 nmol/L; 2.3 ng/ml). Sixteen of them were receiving long-term digoxin therapy, and one had an accidental overdose. The immunoassay yielded significantly higher concentrations (4.1 +/- 1.2 nmol/L; 3.2 +/- 0.9 ng/ml) than the HPLC method (3.3 +/- 1.6 nmol/L; 2.6 +/- 1.2 ng/ml; p less than 0.01). In five cases (30%) these differences were clinically significant because administration of digoxin had been discontinued in the presence of true digoxin concentrations within the therapeutic range and the lack of clinical toxic effects. These data suggest that therapeutic drug monitoring using immunoassays of digoxin may be too inaccurate to detect potential toxic effects, and that much more weight should be focused on clinical monitoring. The HPLC method for assay of digoxin is extremely meticulous and will not become clinically available; therefore the development of better immunoassays should be encouraged.