Complex and frequent ventricular ectopy have been associated with an increased risk of sudden death in certain patient groups. The evaluate the relationship between complexity and frequency of ventricular ectopy, 275 consecutive 20- to 24-hour ambulatory electrocardiograms were analyzed. The unselected study population included 56% males and the average age was 56. Prior myocardial infaraction was present in 28%. Dizziness, palpitations and syncope were present in 16, 30 and 13%, respectively. Complex ventricular ectopy (complexity) was definced as multiformity, bigeminy, couplets or salvos, ventricular tachycardia (VT), and R on T. Premature ventricular depolarizations (PVDs) were observed in 230 of 275 records and exhibited one or more criteria for complexity in 164 (71%). Of the 164 records with complexity, 71% had multiformity, 35% bigeminy, 32% couplets, 5% VT, and 22% R on T. Of 159 records with average PVD frequency less than or equal to 30/h, 98 (61%) exhibited complexity, whereas 66 of 71 (93%) records with greater than 30 PVD/h had complexity. Of 113 records with greater than 100 PDVs/24h, 102 (90%) exhibited complexity, whereas 60 of 99 (61%) records with between 2 and 100 PDVs on the entire record showed complexity. Of the 164 records with complexity, 43 had 'rare PVDs' (less than 30/24 h), but exhibited 72% multiformity, 5% begeminy, 25% couplets, 2% VT, and 14% R on T. This study demonstrates a high prevalence (50-60) of complexity in patients with 'infrequent' PDVs defined as less than 30/24 h, less than 30 average per h, or less than 30 per any 1 h. In approximately 25% of patients with infrequent PDVs, complexity included repetitive ventricular beating (couplets, salvos, or VT). These findings indicate that infrequent PVDs are often complex.