From 1958 to 1969 conizations for carcinoma in situ of the uterine cervix were carried out in 683 women. In 523 cases no further therapy was instituted immediately. Step serial sections of the conization specimens showed that carcinoma in situ was completely removed in 346 (66.2%) of the cases. In 177 (33.8%) of the cases, the excision of the atypical epithelium was incomplete or not with certainty complete, by the conization. Of these patients 43 women were older than 45 years. Follow-up examinations after conization with complete removal of all atypical epithelium showed that only one case showed again a pathological epithelium. In this case, hysterectomy for an adnexal tumor four years after conization showed a mild dysplasia of the endocervix. Follow-up examination of the cases with incomplete or possibly incomplete conization showed no further pathological finding in 90% of the cases. In 19 (10.7%) of the women, atypical epithelium reappeared after varying lengths of follow-up. In 4 women, invasive carcinoma had developed and in 2 women, microinvasive carcinoma had developed. Following operative treatment, further follow-up examinations in these patients were negative. Conization of the uterine cervix with complete removal of all atypical epithelium is shown to be adequate treatment for carcinoma in situ. In cases with incomplete or not with certainty complete removal of atypical tissue by conization, individual follow-up is necessary. If further pregnancies are desired the small risk of observation is justifiable. Diagnosed persistence of atypical epithelium is an indication for a hysterectomy. Hysterectomies are also necessary if the patient does not comply adequately with the follow-up protocol.