During a 14-month period, 75 deaths occurring in relation to 53,581 cardiac catheterizations were consecutively and prospectively reported to the Registry Committee of the Society for Cardiac Angiography. Three of the patients died several days after their catheterization from an unrelated cause and are excluded from this analysis. There were 21 patients (group I) who arrived at the laboratory in extremis and whose deaths were expected irrespective of the catheterization. Most of these patients suffered from recent myocardial infarctions and cardiogenic shock, or had complex congenital malformations. In 35 patients (group II), a cardiovascular complication occurring during the catheterization resulted in death. In 16 patients (group III) catheterization seemed uneventful, but death occurred suddenly 10 min to 10 h after the procedure. Of these 16 patients, eight had left main coronary artery obstruction greater than or equal to 90%, five had three-vessel disease all with 90% obstructions, one had 2-vessel disease both with 90% obstructions, and who had critical aortic stenosis. The 51 unexpected deaths (groups II and III) were considered to be causally related to the procedure, a mortality rate of 0.10%. Subsets with an increased mortality rate (M), were patients with: a) left main disease greater than 50% (M = 0.94%); b) ejection fraction less than 30% (M = 0.54%); c) NYHA class III or IV (m = 0.24%); d) age over 60 years (M = 0.23%); or e) three-vessel disease (M = 0.13%). In conclusion, catheterization related mortality occurs mostly in patients with far advanced cardiac disease. Nearly 1/3 of the unexpected deaths occurred suddenly after a seemingly uneventful procedure. Close monitoring after catheterization of patients with similar characteristics (left main disease greater than or equal to 90%, or three-vessel disease all greater than or equal to 90%) might disclose avenues for reducing mortality occurring after catheterization.