One hundred forty-four patients with coronary heart disease previously resuscitated from out-of-hospital ventricular fibrillation underwent 24-hour ambulatory ECG monitoring 5 months (median) after cardiac arrest. Patients were then followed for an average of 31.7 months. Fifty-one patients died, 32 from subsequent unexpected cardiac arrest. One hundred twenty-nine patients (90%) had uniform ventricular ectopy. Two-thirds had complex ventricular ectopy (Bigeminy/trigeminy, repetitive forms or frequent multiforms). Complex ectopy, present in 20% or more of the 30-minute recording intervals, occurred in 18 of the patients (56%) who developed subsequent cardiac arrest, compared with 26 of 92 (28%) who survived (P less than 0.005, specificity 72%, relative risk 2.3). Other forms of ventricular ectopy, e.g., uniform, multiform and repetitive beats, were more sensitive but less specific predictors of death or recurrent ventricular fibrillation. Complex ventricular ectopy was associated with certain clinical histories: it occurred in 95% of patients with a history of congestive heart failure and in 79% with remote myocardial infarction, compared with 59% and 56% without these histories (p less than 0.001 and p less than 0.01). As expected, mortality was also greater in patients who had these clinical characteristics. Complex ectopy was related to mortality regardless of the clinical history.