The aim of this study was to determine the relative importance of different risk factors in the diagnosis of digitalis toxicity. The authors recruited inpatients for whom serum digoxin level was requested and prospectively followed them for a week to ascertain if they showed digitalis toxicity. The predictive value of different factors for the assessment of digoxin toxicity was analyzed by multiple logistic regression. Forty-one toxic and 58 nontoxic patients were included. In the univariant analysis, intoxicated patients were older, most were women, and they had worse renal function and higher digoxin level; but there were no differences in serum electrolytes or other risk factors. In the multivariant analysis, digoxin level was the only independent factor related to digitalis toxicity. A different risk of toxicity for each clinical manifestation was found for a certain digoxin level. Patients with signs of automaticity in the electrocardiogram had a higher likelihood of being intoxicated than patients with gastrointestinal symptoms, atrioventricular block, or bradycardia. Therefore, in the population evaluated in this study, digoxin level is the key independent factor in digoxin intoxication, although the probability of being intoxicated is also a function of the type of clinical manifestations. A graphic approximation of this probability based on these two factors is presented.