An analysis of 2,394 selective coronary angiograms yielded 23 examples of coronary artery spasm. Of these, nine occurred in patients with Prinzmetal's variant angina and 14 were instances of catheter-induced spasm. Angiographic criteria can distinguish between the spasm of variant angina and catheter-induced spasm. The latter is usually asymptomatic, almost invariably in the right coronary artery, at the catheter tip, smooth, concentric, and less than 2 mm long. The former can occur in any coronary artery at a distance of 1--4 cm from the catheter tip, is usually irregular and eccentric, and is associated with angina, ST segment elevation, hypotension, and dysrhythmia. Response to nitroglycerin is often, but not always, complete in both. Stenoses that seem to be fixed in patients with Prinzmetal's angina should be suspected to be spasm even if unresponsive to nitroglycerin, especially when the rest of the vessel is normal. Additional pharmacologic manipulation and even recatheterization may be necessary to prove the dynamic nature of the lesion and avoid unnecessary surgery.