Height, weight, total body fat, lean body mass, size of fat cells and total number of lipid-containing fat cells were compared in two groups of children, ages 2-6 years, with congenital heart disease. Twelve children who developed heart failure in infancy made up one group and 14 asymptomatic patients made up the other. No patient was cyanotic or had undergone surgical repair. Patients with heart failure were shorter and lighter than asymptomatic patients. Whereas both fat and lean tissues were less in the heart failure group, reduction in body fat was more important in producing underweight. Although there was no difference in the size of fat cells, the number of lipid-laden fat cells was diminished in the children who had early heart failure. Thus hypocellularity of fat cells appeared to be the major determinant of diminished body fat with heart failure. Because shortness and decreased lean tissue were associated with these adipose tissue abnormalities, it is likely that widespread changes are caused by early heart failure. As previously reported with hypoxemia, the early development of heart failure seems to produce tissue changes that handicap growth.