Plasma triglycerides, total cholesterol and high-density lipoprotein cholesterol were studied in patients with Alzheimer's disease (AD, n = 57, mean age 70 years) and multi-infarct dementia (MID, n = 69, mean age 73 years) when the patients were admitted for assessment. Both total cholesterol and high-density lipoprotein cholesterol but not triglycerides were lower in MID than in AD even though there was a considerable overlap. Especially in younger patients and in patients living at home the difference was not statistically significant. Further, the plasma lipid values in neuropathologically confirmed cases with AD (n = 5) and MID (n = 16) were similar at admission. Low total cholesterol and high-density lipoprotein cholesterol were related to cardio- and cerebrovascular disorders, living in institutions, and negatively correlated to age and severity of dementia. Our results suggest that determination of total cholesterol and high-density lipoprotein cholesterol is of minor value in the differential diagnosis between AD and MID and that associated diseases, such as coronary heart disease, cardiac failure and arterial hypertension, are more important in this respect.