Prognosis of patients with episodes of hypotensive ventricular tachycardia (VT) or ventricular fibrillation (VF) in the absence of structural heart disease is poorly defined. To solve this problem, this study analyzed a subgroup of 25 such patients chosen from 468 consecutive patients who had an initial implantable cardioverter defibrillator (ICD) inserted between May 1984 and May 1990 in 9 medical centers and were followed up for at least 1 year. The patient group consisted of 17 men and 8 women, aged 8 to 75 years. Cardiac arrest occurred in 20 patients, 3 patients had recurrent VT, and 2 patients had both. Left ventricular ejection fraction ranged from 50% to 70%. During electrophysiologic study, a specific response was seen in 13 patients, defined as monomorphic VT (5 patients), or VF in those who had a history of VF (8 patients). In 8 patients, only a nonspecific response was seen. No arrhythmia could be induced in 4 patients. Of the 13 patients with a specific response, antiarrhythmic drug was tested in 9; in 3 of them the arrhythmia was suppressed. Within the first year, 6 of the 25 patients (24%) received appropriate shock. In the remaining 436 patients who had organic heart disease, 155 (36%) received appropriate ICD shock (p = NS). Therefore, ICD implantation appears to be warranted in patients with a history of life-threatening arrhythmias, not only in the presence but also in the absence of demonstrable structural heart disease.