The myofibroblast, a cell with features of both fibroblasts and smooth muscle cells, was described originally in granulation tissue. Since then it has been seen in various neoplastic and nonneoplastic conditions. However, there are still unanswered questions regarding the origin of the myofibroblast. Although the cell's derivation from fibroblasts seems clearly established, it has been proposed that these cells arise from the peritoneal macrophages around free-floating intraperitoneal blood clots in rats. We have studied a case of juvenile xanthogranuloma by light and electron microscopy. The histologic features were typical, although the Touton type of giant cells were uncommon. Electron microscopy showed lipid-containing macrophages; many of these also contained the transversely banded bundles of fibrils characteristic of myofibroblasts. These findings support the macrophage origin of myofibroblasts in humans.