Intraoperative arrhythmia activation mapping underlies the impressive success of surgery for sustained uniform ventricular tachycardia. Unstable arrhythmias and those intraoperatively noninducible, however, are not amenable to activation mapping and strategies for dealing with them are poorly defined. We propose that fragmentation mapping in sinus rhythm can be used to direct surgery in such situation. In 21 (33%) of 64 patients operated upon at this unit, intra-operative arrhythmia mapping was impossible because of non-inducibility in 17 (27%) and unstable morphology in 4 (6%). Endocardial resection was performed in all areas showing 'fragmented' local electrograms (greater than 100 ms duration at 30-300 Hz filtering). Mean patient characteristics included: age 51 years; LV ejection fraction 32%; major arrhythmic episodes 16 (range 2-200); antiarrhythmic drug failures, 4. There were 5 (24%) early postoperative deaths (heart failure 3; sudden 1; metabolic 1) and 1 early arrhythmia recurrence. There were 3 late non-arrhythmic deaths and 1 further arrhythmia recurrence during follow-up of 23 +/- 19 months. Both patients with documented postoperative arrhythmic episodes were controlled on previously ineffective antiarrhythmic drug therapy. Fragmentation mapping in sinus rhythm successfully extends the surgical option to arrhythmias previously considered inoperable. The results compare favourably with those for arrhythmias in which surgery was directed by activation mapping.