The aim of this study was to determine the extension of cervical intraepithelial neoplasia grade III (CIN III) into endocervical canal and depth of endocervical crypts involvement by CIN with the regard to patients' age and parity. Correlation between the area of CIN involvement and the extension into endocervical canal was estimated. A total of 218 cervical cone specimens with histologically proven CIN III were included in this study. Extension of CIN into the endocervical canal, depth of involved crypts and ectocervical area affected by CIN were histologically analyzed. The average endocervical crypt involvement was at 1.2 mm of depth. The excision of >4 mm (1.2 mm x 3S.D.) in depth removes >99% of CIN. With the cone length of 15 mm (nulliparous patients) and 18 mm (multiparous patients), no endocervical cone margins were affected with CIN. Since the cone length is the most important determining factor for fertility preservation, the measurement of cervical cone could be essential for future pregnancies.