Knowledge of the frequency and extent of crypt involvement by cervical intraepithelial neoplasia (CIN) is important when treatment by electrodiathermy, cryosurgery, or the laser is considered. Three hundred forty-three therapeutic cone biopsies carried out at The Samaritan Hospital for Women, London, have been reviewed histologically. Measurements were made of the deepest crypt in each specimen and of the deepest crypt that contained CIN. The mean depth of involved crypts was 1.24 mm, and the mean depth of uninvolved crypts was 3.38 mm. Thus, destruction of tissue to a depth of 2.92 mm (mean + 1.96 SD) would eradicate all involved crypts in 95% of patients, whereas destruction to a depth of 3.80 mm would eradicate 99.7% (mean + 3 SD). There is also an increase in the depth of crypt involvement with increasing age, but the differences between the age groups are not statistically significant.