To estimate the frequency of potentially life-threatening arrhythmias in myocardial infarction following transfer from the coronary care unit (CCU) and to identify features of the acute illness which predict such events, 66 patients were monitored on-line by means of a computer assisted system. Premature ventricular contractions (PVCs) were detected following transfer from the CCU in 64 patients (97%). In 29 (44%) they fell in classes 2 to 4 of Lown. Accelerated ventricular rhythm was detected in five and ventricular tachycardia in three. The presence of these rhythm disturbances did not correlate with age, sex, infarct location, the occurrence of previous infarction, the level of serum cardiac enzymes, the presence of heart failure, atrial arrhythmias, heart block, or serious ventricular arrhythmia in the CCU. Use of procaine amide or quinidine for persistent ventricular arrhythmia in the CCU was correlated with detection of class 2, 3 or 4 PVCs. Thus, PVCs are nearly universal in the late phase of hospitalization for myocardial infarction. Frequent and complicated PVCs are common and occur most frequently in individuals in whom such events have been persistent in the CCU.