Detailed chemical analyses have been carried out on aorta, and on coronary and aortic atheromata, of men who died during a study of prolonged use of diets rich in unsaturated fat and of control subjects. Concentrations of total aortic lipid and of total aortic calcium were not significantly different for the two groups of subjects. Concentrations of cholesterol and cholesterol esters, triglyceride, and phosphatide showed no difference between the two groups in analyses of aorta, uncomplicated aortic atheromata, complicated aortic atheromata, and coronary atheromata. Coronary atheroma lipid contained substantially more triglyceride than did aortic atheroma lipid, whether derived from complicated or uncomplicated plaques.Atheroma triglyceride in experimental subjects contained more linoleic acid than in control subjects, in all types of plaques. Lesser but significant increases in linoleic acid were seen in cholesterol ester and phosphatide of coronary atheromata and complicated aortic atheromata. In the case of uncomplicated aortic atheroma, changes in linoleic acid of cholesterol ester and phosphatide were smaller and not significant. In both groups of subjects, coronary atheroma contained more cholesteryl oleate and less linoleate than did aortic atheroma.Linoleic acid contents of atheroma lipid fractions were positively correlated with the linoleic acid figures for corresponding fractions of the most recent antemortem serum samples. Ratio of linoleic acid in an atheroma lipid fraction to the linoleic acid content of the corresponding serum fraction was, in most cases, constant after the time of the first sample, obtained after 495 days on experimental diet. It is postulated that this ratio constitutes an estimate of the fraction of atheroma fatty acid derived from plasma. In cholesterol ester and triglyceride, the ratio was about 0.65 in experimental subjects, higher in subjects on the control diet. The ratio for phosphatide was about 0.5 in both groups of subjects.