• The diagnostic procedure here recommended is based on two series of studies. Each involved 100 hysterectomies in which the removed uterus had been found to have cervical carcinoma in situ. In the first series, initial biopsy had failed to detect the carcinoma in 26 instances, and in most of these the biopsy specimens were not considered representative because the surface epithelium was absent or the epithelial changes were minimal. In the second series, the location and form of the carcinoma were reconstructed from the sections of tissue removed, and the inadequacies of the biopsy technique were at least partially explained by the preponderance of lesions in the cervical canal. The procedure recommended therefore begins with cytological studies of the cervix. If these indicate the need for further study, a representative biopsy specimen must be obtained. The technique of sharp-knife or cold conization avoids tissue trauma, delineates the excision area by iodine staining, gives an adequate length of excised cervical canal, and is followed by frequent dilations of the cervix to guard against stenosis. This excision is not a definitive treatment for carcinoma in situ, for about two-thirds of subsequent hysterectomy specimens show either carcinoma or dysplastic epithelium, but it yields satisfactory diagnostic material.