The records of 208 women were reviewed to quantify the long-term treatment failures of large loop excision of the transformation zone (LLETZ) in the treatment of cervical intraepithelial neoplasia (CIN) grade III. The rate of dyskariotic smears fell from 8% at 6 months to 2.3% at 48 months. The excision was considered incomplete in 84 cases (40.3%) of which 40 had endocervical margins involved. Thirty-five per cent of women with incomplete excision of CIN at the endocervical margin and 9% with complete excision had a dyskariotic smear within 4 years following LLETZ. These findings were statistically significant (P = 0.003 and 0.02 respectively). However, 5% only of women in the former group had cytological treatment failure at 6 months. There was no significant association between age and incidence of treatment failure. Further data are required to determine the risk of recurrence beyond 4 years before recommendations could be made about the duration of annual screening.