A series of 1121 patients with carcinoma in situ have been followed for 5 to 25 years. Recurrences of in situ lesions and development of invasive cancer wwere found, often many years after treatment. Thereapeutic conization was performed in 795 patients, of which 19 (2.3%) had recurrent carcinoma in situ and 7 patients (0.9%) developed invasive cancer. The corresponding figures for 238 patients treated with hysterectomy were, respectively, 3 (1.2%) and 5 (2.1%). The invasive lesions appeared after treatment several years later than the in situ lesions. It is stressed that women having once had in situ carcinoma of the cervix will always be at some risk, and therefore should be carefully followed for a much longer time than the conventional 5 years. Of 42 cases of preinvasive carcinoma extending to the border of the surgical specimen in which conization was performed, 25 were not immediately treated but only observed. Four of these patients developed recurrence from 2 to 6 years after treatment, while 21 are well after a followup period of between 5 and 15 years.