Background: Cold-knife conization of the uterine cervix is a diagnostic and often curative procedure in patients with intraepithelial neoplasia of the uterine cervix. Patients in whom conization has not removed the lesion completely may require further treatment. Patients and Methods: Between 1970 and 1990 3,663 patients with carcinoma in situ of the cervix underwent cold-knife conization. In 669 patients (18%) histology showed that the lesion extended to the margins of the conization specimen. 441 of these patients, especially younger ones who wished to preserve their fertility, were followed up observantly and 228 patients underwent hysterectomy. Results: After incomplete conization a total of 6 out of 669 patients (0.1%) developed recurrent disease. Four patients (1.8%) developed grade 3 vaginal intraepithelial neoplasia after hysterectomy and 2 patients (0.5%) who were followed up observantly showed persisting carcinoma in situ. Of the 228 uteri extirpated immediately after incomplete conization, only 69 (30%) showed residual disease. No patient developed invasive disease. Conclusions: The results suggest that patients with incomplete conization for carcinoma in situ of the cervix can be followed up observantly with little risk of recurrence.