In a study of the relation between ventricular premature beats and sudden death among 1,739 male of myocardial infarction enrolled in the Health Insurance Plan of Greater New York (HIP), patients underwent 1 hour of electrocardiographic monitoring at a baseline examination. During follow-up periods of up to 5 1/2 years, survivors underwent repeated monitoring at 6 month intervals for a maximum of four monitorings. At each monitoring a constant proportion of the men--25 percent--showed complex ventricular premature beats (runs of two or more, R on T phenomenon, bigeminal or multiform beats) during the hour. In comparison with men free of such arrhythmia, those demonstrating these complex forms in a given hour were three times as likely to show such beats in a subsequent monitoring hour. The mortality risk over 3 1/2 years after each of the four monitoring observations was in all cases elevated among men with complex ventricular premature beats. The risk of sudden death over this period was 6 percent for men without and 13 to 17 percent for men with such complexes. A study of the 1,445 men who underwent monitoring both at baseline examination and 6 months later identified the presence of runs of ventricular premature betas in either observation as a particularly important harbinger of sudden death.