Four patients with a primary histological diagnosis of Bowen disease of the nail bed were treated by the Mohs chemosurgery technique. The presence of invasive squamous cell carcinoma was disclosed during chemosurgery in three cases. The fourth case had already progressed to invasive squamous cell carcinoma before treatment. It is urged that biopsies be performed in all cases of persistent and recurrent disease of the nail bed so that an early diagnosis may be made and treatment instituted before the development of invasive carcinoma. Mohs chemosurgery permits adequate excision of the tumor with maximal preservation of normal tissue and function.