In a series of 1,080 consecutively monitored patients who received digoxin, arrhythmias attributed to the drug were observed in 144 cases (13.3%). The proportion of arrhythmias among 510 patients with blood group O was 10%, while for the 570 patients of groups A, B, and AB (non-O), it was 16.3% (P< 0.002). The point estimate of relative risk for non-O patients relative to group O patients with 1.7, with 95% confidence limits of 1.2 and 2.5, and the increase in the risk was most prominent between the ages of 50 and 69 years.