The serum digoxin concentration (SDC) was measured in 100 consecutive patients with suspected digitalis-induced arrhythmias. In 24 (toxic group) of these subjects arrhythmias disappeared after glycoside withdrawal. The mean (+/- SD) SDC was 2.88 +/- 1.89 ng/ml in the toxic group and 1.00 +/- 0.61 in the nontoxic group (p less than 0.0001). With respect to digitoxicity, the SDC showed a predictive accuracy rate of 88%, a specificity rate of 89%, and a sensitivity rate of 62%. Since our results were obtained from symptomatic patients, we observed a better predictive accuracy rate than other authors, who studied symptomatic and asymptomatic subjects. The high percentage of false-negatives makes it impossible to consider the SDC by itself a diagnostic test for digitoxicity. However, the SDC is a useful test since all other parameters are less sensitive and/or less rapid indicators of probable digitalis intoxication.