A case of ventricular fibrillation occurring in the recovery phase of treadmill exercise test is described. A 64-year-old man was admitted to the hospital because of chest pain. The resting electrocardiogram was normal, but during a stress test (Bruce protocol) ventricular ectopic beats and ischemic S-T depression were observed. The echocardiogram showed septal hypertrophy and septal and apical hypokinesia. Ten days later he, during antianginal therapy, underwent treadmill exercise testing. In contrast with the previous stress test, ventricular ectopic beats decreased during the effort, but the S-T segment depressed, at the third step, with mild chest pain and hypotension. While angina and electrocardiogram improved, suddenly ventricular fibrillation developed. The patient was successfully resuscitated with electrical defibrillation, with a 350 joules shock. Echocardiogram was unchanged; serum enzymes were slightly modified (secondary to DC-shock). The ischemic changes observed at the electrocardiogram in the anterior wall within seven days returned in the normal range. Ten days later the patient underwent cardiac catheterization. At ventriculography the posterior and diaphragmatic wall was diskinetic, ejection fraction was 70%; a complete occlusion of the right coronary artery at the origin, a critical stenosis of left descending artery, with occlusion in the middle tract were found. Circumflex artery was occluded at the origin with omo-etero-coronary collateral channels. Twenty days later, he had a successful bypass surgery. Postsurgical echocardiogram revealed a moderate depression of ejection fraction (41%), inferior diskinesia, apical hypokinesia, mild mitral regurgitation. The patient was discharged with digitalis, nitrates, aspirin and amiodarone for prophylactic treatment of paroxystic atrial fibrillation.(ABSTRACT TRUNCATED AT 250 WORDS)