More and more stress has recently been laid on the importance of early carcinomas which have not as yet broken out of their parent epithelium to invade surrounding tissue and are hence known as "carcinomas in situ." At present, carcinoma in situ is well recognized as a type in the breast, cervix and stomach. The term is beginning to be applied to Bowen's "precancerous dermatosis" of the skin and to exactly similar lesions in the esophagus and anus. Carcinoma in situ is readily detected in biopsies of the breast and cervix uteri, provided that the surgeon has been fortunate enough to obtain specimens coinciding with the affected areas. In the skin it is usually easily diagnosed on gross inspection, and biopsy is simple and conclusive. While this is also in a measure true of such tumors in the anal canal, it is manifestly difficult or impossible to obtain biopsy specimens