The surgical therapy of hypertrophic cardiomyopathy (HOCM) has been shown to improve symptoms and the haemodynamic status. The prognosis after transaortic subvalvular myectomy seems to be relatively better compared to those patients with HOCM who undergo medical therapy. Complex ventricular arrhythmias have been shown to influence prognosis. Therefore, a study was undertaken to analyse the influence of surgery on ventricular arrhythmias in patients with HOCM. Thirty-one patients (23 male, eight female), mean age 44 +/- 13.6 years (range 13 to 72 years) underwent 48 h ambulatory monitoring pre- and postoperatively. Additionally 15 patients with hypertrophic non-obstructive cardiomyopathy (HNCM) were studied. Mean age was 49 +/- 11.5 years (range 28 to 69 years). Complex ventricular arrhythmias were defined as pairs (two consecutive QRS complexes) or non-sustained ventricular tachycardia (VT) (much greater than 3 QRS). Results in the operative group (HOCM) were as follows: The overall frequency of ventricular extrasystoles was low. There was no difference in the mean hourly ventricular extrasystole counts pre-operatively as compared to postoperatively. Thirteen patients had no pairs or VT pre- and postoperatively. Six patients had VT or pairs pre- and postoperatively. In five patients complex arrhythmias were detected only before surgery, whereas in another seven patients VT was first documented after operation. Overall, 11 patients had complex ventricular arrhythmias pre-operatively (pairs n = 3; VT n = 8, 3 to 8 QRS, rate 142 +/- 15 beats/min). Thirteen patients had complex ventricular arrhythmias after operation (pairs n = 4; VT n = 9, 4 to 10 QRS, rate 150 +/- 40 beats/min). Results in patients with HNCM were as follows: The overall frequency of ventricular extrasystoles was low, only three patients had a mean extrasystole count of more than 20 beats/h. Five patients had pairs and three patients ventricular tachycardia (5 QRS, rate 110 to 160 beats/min).(ABSTRACT TRUNCATED AT 250 WORDS)