Background: Onychomatricoma is a nail matrix tumor that has been well characterized clinically but not histologically. Objective: The purpose of this study was to establish histologic criteria for the diagnosis of onychomatricoma to differentiate it from other fibroepithelial tumors of the nail matrix. Methods: We observed 12 cases and were able to examine 4 of these excised specimens, including the attached nail plate. In 8 patients, avulsion of the nail was performed before excision of the tumor so that the nail plate was examined separately from the tumor. Results: Onychomatricoma is a fibroepithelial tumor consisting of 2 anatomic zones. The proximal zone is located beneath the proximal nail fold with a proximal border starting at the root of the nail and distal border corresponding to the cuticle. It is characterized by deep epithelial invaginations filled with a thick V-shaped keratogenous zone, a thickened nail plate without cavitation but with an undulating inferior border ending in ungual spurs, and a fibrillary stroma clearly demarcated from the undersurface. The distal zone corresponds to the lunula and is characterized by multiple “glove finger” digitations lined with matrix epithelium and oriented around antero-oblique connective tissue axes; perforation of the nail plate by multiple cavities that, generally at the distal edge of the lunula, lose their epithelial digitations and become filled with serous fluid; the connective-tissue stoma of the digitations extends deeply into the dermis and is not demarcated form healthy tissue. Conclusion: On the basis of the mentioned characteristics, we have been able to define onychomatricoma histologically. In addition, we have identified an unusual clinical form of onychomatricoma that has the appearance of a cutaneous horn and is situated at the junction of the undersurface of the proximal nailfold and the lateral nailfold. (J Am Acad Dermatol 1998;39:560-4.)