Electron microscopy of the skin was performed in ten patients with systemic and four with localized scleroderma. The following three groups of cells were identified: (1) mature lymphocytes, T lymphoblasts, immature plasma cells, and plasma cells; (2) fibroblasts, fibrocytes, and fibroblast-like cells; and (3) macrophages, undifferentiated mesenchymal cells, and monocytes. Inthose specimens with mononuclear cell infiltrates, the most common cells were lymphocytic-types, macrophages, and fibroblasts with well developed rough endoplasmic reticulum. In the specimens at the fibrotic stage, fibroblasts and histiocytic-type cells predominated. This study suggests that cellular and humoral immunity may play a role in the pathophysiology of scleroderma.