Histologic, histochemical, and ultrastructural studies of a tuberous xanthoma from a patient with homozygous type II hyperlipoproteinemia showed that all of the lipid was within histiocytic foam cells; no lipid was identified in interstitial regions or in blood vessels. Primitive mesenchymal cells, elongated perivascular and fibroblast-like cells, and lysosome-filled macrophages also were present within the xanthoma, indicating possible stages in the evolution of dermal mesenchymal cells into mature, cholesterol-rich foam cells. Morphologically, the lipid was in four different forms: large droplets, which were the dominant form, and membrane-bound crystals, concentric lamellar bodies, and ceroid. The paucity of membrane-bound lipid forms, relative to the abundant free lipid droplets, indicated that lysosomal digestion was a minor metabolic pathway for the intracellular metabolism of lipid in the xanthoma. Thus, nonlysosomal lipid storage in foam cells is a characteristic tissue response to the underlying metabolic defect in type II hyperlipoproteinemia.