Studies of the tissue lipids of two siblings with Wolman's disease are presented. In both patients massive accumulations of triglycerides, as well as of free and esterified cholesterol in the liver and spleen were found. Linoleic acid was greatly reduced in the tissue lipids of one patient who received a skim milk diet, but was present in normal concentrations in the second, whose diet was based on a soybean formula. Plasma α-lipoproteins were absent by electrophoresis in the first patient at age 6 months, but were present at birth in the second patient, though they disappeared as the disease progressed. Cultured fibroblasts from two patients showed increased content of cholesterol, and increased incorporation of mevalonic acid into cholesterol in one of these.