Lipid analysis of different constituents in the blood of patients with stroke revealed that an increased level of total serum cholesterol was infrequently (that of triglycerides more frequently) associated with stroke in middle-aged men, whereas the high-density lipoprotein cholesterol content was low in all but young patients with stroke. High contents of cholestanol and low contents of n-6 polyunsaturated fatty acids predict the development of stroke in middle-aged men and are found in both middle-aged men with stroke and elderly patients with multi-infarct dementia. Young patients with unexpected stroke had normal serum sterol, triglyceride, and fatty acid contents but markedly reduced arachidonic acid levels in the platelets, a change that was normalized by treatment with aspirin. The results emphasize that the etiopathogenesis of ischemic brain damage may vary markedly from one age group to another.