Although greater than in patients with stabilised coronary insufficiency, the risks of carrying out coronary arteriography during the period of a threatened infarction are still modest in the hands of an experienced team. In a series of 66 such investigations under these conditions over an 18 month period, there were no deaths as immediate sequelae. Three myocardial infarctions occurred, one of which was probably encouraged by ceasing the propranolol prior to arteriography. Analysis of the results confirms the frequency of lesions affecting all three trunks (47% of cases), the anterior descending branch being the most commonly affected. A collateral circulation was established in 23% of cases. Ventriculography was normal in one third of cases. The mean value for the ventricular ejection fraction was 0.57 over the entire series. In 58% of cases, all the arteriographic and ventriculographic criteria of operability were satisfied. It therefore appears that coronary arteriography is justifiable during the period of threatened infarction; it may be undertaken at an early stage if the angina does not respond to medical treatment. It is always best to carry it out several days after cessation of the pain where possible. The investigation may lead to urgent surgical intervention.