Elderly institutionalized men were assigned at random to two groups, one of which received a conventional diet while the other was fed a diet in which the major modification was substitution of unsaturated for saturated fat. Changes in serum lipids and in adipose tissue over periods up to 5 years are described. In control subjects, mean serum cholesterol rose 4% over the first 20 months, then fell during the next 40 months to a level 10% below the starting concentration. In the experimental group there was an immediate drop, followed by further changes roughly parallel to those in the control subjects. The mean difference between the control and experimental groups was 14.0% of the starting level. Changes in serum total lipid were similar, but the percentage difference between control and experimental groups was only 6.8% of the baseline level. All major esterified serum lipid fractions of experimental subjects contained increased concentrations of linoleic acid. This was most marked in triglyceride, which at 3 years had a composition similar to that of the dietary fat in both groups of subjects. Adipose tissue linoleic acid rose in men on the experimental diet from 11% of total fatty acid at time zero to 32% at 5 years. The rise could be fitted to an exponential function with a half-time of 680 days. The rate of rise during the 1st year was correlated negatively with initial body weight and positively with weight gain; the influence of adherence to the diet was much less pronounced.