During 18 months, 25 asymptomatic apparently healthy subjects were consecutively studied because of electrocardiographic discovery of frequent (more than 3 beats/min) or complex (multiform, alternating or repetitive patterns) ventricular ectopy of unknown cause. History, physical examination, laboratory data, chest roentgenogram, vectorcardiogram, exercise testing, echocardiogram, His bundle electrogram and cardiac catheterization procedures were performed to detect cardiac abnormalities. Ventricular ectopy was studied with maximal exercise testing, 24 hour Holter recordings and antiarrhythmic drug regimens. The duration of ventricular ectopy ranged from 1 to 30 years (mean 6). Eighteen subjects had an apparently normal cardiac status, and seven had mild or suspected abnormalities. Ventricular ectopy was of predominantly right ventricular origin in 19 of 25 subjects, and disappeared during maximal exercise in 21 of 23 subjects. Characteristic frequent (mean 559 beats/hour) and complex (bigeminal, trigerminal or ventricular couplet) ectopic beats were consistently present in all subjects during a 24 hour diurnal cycle. Antiarrhythmic drug regimens were generally ineffective. Our data suggest that a clinical syndrome of frequent complex ventricular ectopy can occur in apparently healthy subjects. The etiology and natural history of this syndrome are undefined.