To find out the types of atherosclerotic plaques that fissure and where they fissure, plaques from 85 patients who had died from coronary thrombosis were examined histologically. 67 of the plaques contained an eccentric pool of extracellular lipid in the intima; 42 (63%) of these plaques had fissured at the junction of the plaque cap with the more normal intima, and the other 25 (37%) had torn through the centre of the cap. Computer modelling of different forms of plaque showed that at systole eccentric pools of lipid concentrated stress on the plaque cap, especially near the edge of the plaque. When the lipid pool occupied less than 15% of the vessel circumference, and when the plaque cap was less stiff than the adjacent normal intima, the point of maximum stress was over the centre of the plaque. Computer modelling also showed that the distribution of circumferential tensile stress across the intima was radically altered by atherosclerotic plaques. Regions of high circumferential stress correlated well with the site of intimal tears found at necropsy. The histological findings showed that site of tearing was influenced by variation in the mechanical strength of cap tissue due to focal accumulation of foam cells. Focal weak points in the cap would explain tears which were not at the point of maximum stress.