A study of the association of ventricular arrhythmias with the presence or absence of significant symptomatology in a predominantly ambulatory patient population. Dynamic electrocardiograms of 10-hr length were recorded, on 1421 occasions, from 1108 patients, a majority of whom were men and women aged 40 to 80 years. Detailed statistical analysis of the ventricular arrhythmias encountered in these recordings is presented. Paroxysmal ventricular tachycardia (PVT), an extremely hazardous arrhythmia, occurred in 50 patients (4.5 percent); the incidence increased with increasing age. PVT increased where unifocal ventricular premature contractions were demonstrated at a frequency of more than 12 per minute, or multifocal ventricular premature contractions of any frequency. PVT increased where patients demonstrated unifocal or multifocal paired ventricular premature complexes of any frequency, but did not occur with supraventricular arrhythmias alone. Only 4 of the 50 patients demonstrating PVT were symptomatic. 26 pp. Bibliog.