The sclerotic fibroma (SF) is a tumor of the skin that may occur sporadically or in the context of Cowden's syndrome. The authors studied four examples of this tumor in an effort to understand better the nature of the fibrous matrix in SF and its significance. A novel antibody was utilized that is directed against the immunoreactive amino-terminal precursor domain of human type I procollagen (AP). This peptide is usually identified only at sites of active or recent collagen synthesis and deposition. All examples of SF showed strong cellular and stromal staining for AP. The matrix also demonstrated variable staining for type IV collagen and laminin. The overall immunophenotype of this lesion suggests that it may be a specialized fibroblastic ('dermal dendrocytic?') tumor. Despite its hypocellularity and hyalinized nature, SF is thought to exhibit ongoing matrical production that supports a neoplastic character for this lesion.