Thirty cases of early stage adenocarcinoma, 5 mm or less in depth, were selected from 1942 primary carcinomas of the uterine cervix for histologic analysis to clarify their histogenesis. There were 16 carcinomas of endocervical type, 12 of endometrioid type, and 2 clear cell carcinomas. There were 22 early invasive adenocarcinomas and 8 adenocarcinomas in situ. In 27 cases the carcinoma was adjacent to the transformation zone and in 3 it was separate from it. In 10 cases adenocarcinoma coexisted with in situ squamous cell carcinoma. Only one patient whose tumor was 3 mm in depth developed a pelvic recurrence after radical hysterectomy. All other patients remained disease-free after treatment by hysterectomy. It is suggested that most adenocarcinomas of the uterine cervix originate from endocervical glands adjoining the transformation zone and that they may develop directly from normal-appearing epithelium without passing through adenomatous or dysplastic changes.