This report describes the case of a 69-year-old man with adenocarcinoma of the transverse colon who developed heterotopic ossification in a metastatic axillary lymph node. The areas of pathologic bone formation were characterized by the appearance of osteoblast-like cells at the surfaces of the mineral deposits. Immunostaining for alkaline phosphatase revealed a significant concentration of this enzyme in these cells and, to a lesser degree, on the apical membrane of the glandular cells of the adenocarcinoma adjacent to the ossification centers. Proliferating mesenchymal cells in close proximity to the areas of osteogenesis also showed significant immunolabeling. We conclude that metastatic colonic carcinoma can promote heterotopic ossification, and that alkaline phosphatase is intimately associated with bone formation under these pathologic conditions.