Left main coronary artery disease i has recently received considerable emphasis with regard to diagnosis, prognosis, and treatment.<sup>1-7</sup>Coronary artery spasm commonly occurs in the right coronary artery system<sup>8-20</sup>and more recently has also been described in the left anterior descending and circumflex coronary arteries.<sup>21-25</sup>Spasm of the left main coronary artery has not, to our knowledge, been documented previously. We describe two patients who had spasm of the left main coronary artery during diagnostic coronary arteriography. Spasm of the left main coronary artery must be considered in the differential diagnosis of left main coronary artery lesions. <h3>PATIENT SUMMARIES</h3><h3>Patient 1.—</h3> Ten days prior to hospital admission, a 57-year-old woman had a near syncopal episode associated with precordial chest pain that was typical of ischemic cardiac pain. She had known of a heart murmur since age 20. There were no other symptoms. She had a blood pressure