Lipoid proteinosis, a rare autosomal recessive disease, is histologically characterized by deposition of hyalinlike material in the dermis. In this study the pathologic processes of lipoid proteinosis were evaluated by ultrastructural and biochemical analysis of skin and cultured fibroblasts from a patient with classic features of the disease. Transmission electron microscopy revealed the presence of hyalinlike material with a granular appearance interspersed between collagen fibers. Immediately surrounding the blood vessel walls, there was reduplication of basal laminae in an “onionskin” arrangement. The fibroblastic cells in the affected dermis contained peculiar cytoplasmic inclusions. Biochemical studies with the cultured fibroblasts showed that the total synthesis of extracellular matrix components, as detected by the synthesis of radioactive hydroxyproline or the incorporation of ³⁵SO₄^2- and [³H]glucosamine into macromolecules, was not altered in lipoid proteinosis. However, the relative expression of type I and type III procollagen genes, as detected by molecular hybridizations with pro-α1(I) and pro-α1(III) procollagen complementary deoxyribonucleic acid probes, was markedly altered in cultured fibroblasts. Specifically, the type I procollagen messenger ribonucleic acid (mRNA) levels were significantly reduced, resulting in a decreased type I/III procollagen mRNA ratio. Furthermore, the replicative capacity of lipoid proteinosis fibroblasts, as detected by the incorporation of radioactive thymidine, was reduced. Thus the skin fibroblasts from lipoid proteinosis demonstrate ultrastructural changes, as well as alterations in their phenotypic characteristics, and these changes may have relevance to the pathologic processes of this systemic disease affecting the skin and other organs.