In 186 patients with Down's syndrome (age 1-68 yr) total cholesterol and triglycerides were determined, quantitative lipoprotein electrophoresis was performed and cholesterol fractions and beta/alpha-lipoprotein ratios were calculated. A reference group of 51 non-mongoloid mentally handicapped adults living in the same institutions were examined in the same way. Patients' total cholesterol, beta-cholesterol and triglycerides did not differ from the controls. However, alpha-cholesterol was significantly lower and the beta/alpha-lipoprotein ratio significantly higher in patients, findings which are associated in the general population with a high risk for premature atherosclerosis. Interestingly, mortality causes and pathological findings in Down's syndrome show no increased frequency of cardiovascular diseases.