A desmoid tumor occurring in the abdominal wall of a 6‐year‐old boy with extensive microcalcifications was documented. Two forms of calcifications were observed: an irregular, amorphous dystrophic calcification of the collagen fibers and an organized, laminated psammomatous calcification (psammoma bodies). Ultrastructural studies revealed that the dystrophic calcification is a result of direct mineralization of individual collagen fibrils with subsequent coalescence to form large, calcified masses. Psammomatous calcification tended to cluster around areas where the collagen fibers assumed a whorled configuration. It is suggested that the circular orientation of the collagen fibers in some way favors the laminar deposition of hydroxyapitite crystals. The cause of the pathologic calcifications is the presence of extensive amorphous eosinophilic degeneration of the collagen fibers.