We studied 22 patients with a histopathological diagnosis of severe dysplasia and 89 patients with a diagnosis of carcinoma in situ of the uterine cervix. Twenty patients (18 per cent) had negative cervical smears for four to six years after diagnosis by biopsy of the cervix. The 91 patients with persistently abnormal smears were treated by cryosurgery, using a double freeze technique with nitrous oxide. Seventy-four (86 per cent) had persistently normal cervical smears after one treatment and eight after a second treatment while nine patients required conization for recurrently abnormal cervical smears. The duration of follow-up was five or six years in 48 patients (53 per cent). In a subsidiary study, material was obtained for histological examination from 72 patients with normal smears by endocervical curettage and multiple biopsies of the cervix; 8 showed slight atypia and 2 had residual carcinoma in situ.