To examine the incidence of a positive indium-111 antimyosin scintigraphy in patients with unstable angina, we prospectively examined 25 patients with unstable angina and 11 patients with stable angina. All patients were injected with 2 mCi of indium-111 on admission and planar scintigraphy was performed 48 h later. Symptoms and signs of ischemia indicating severity of ischemia were monitored during the period between injection and scanning. None of the patients developed a rise in cardiac enzymes suggesting myocardial necrosis. Seven (28%) of the 25 patients with unstable angina had positive antimyosin scanning; no stable patients had positive scanning. All seven patients with unstable angina and positive scanning had signs of severe ischemia (four patients had multiple episodes of > 2/day, three patients had prolonged episodes of > 15 min, three patients had ST depression in > 3 EKG leads) while only one of the eighteen unstable patients with negative scannings had signs of severe ischemia (P < 0.001). In conclusion, (1) a significant number of unstable angina patients present positive antimyosin scanning without an elevation of cardiac enzymes and (2) these patients usually present multiple or severe episodes of ischemia indicating that during these episodes, minor myocardial necrosis, undetected by enzymes, may occur.