Summary. A retrospective histological study was performed on 23 patients who had presented with recurrent cervical intraepithelial neoplasia (CIN) 18 months or more after apparently adequate local destructive treatment and a comparable control group of 23 patients who had no recurrence at 18 months. All recurrences were in women with CIN 3. Detailed examination of pretreatment biopsies showed that there were significant differences in both the extent of crypt involvement (P<0.01) and maximum depth of CIN (P<0.01) between the recurrent groups and the comparison group of women without recurrence. There was also a significant difference in mitotic count between the two groups (P<0.01). Immunocytoehemical staining for Langerhans' cells or human papilloma virus common antigen showed no significant difference between the two groups. A mitotic count of 35 per ten high-power fields or a depth of crypt involvement > 1.7 mm in the pretreatment biopsy was invariably associated with recurrence and identified 13/22 (59%) cases of recurrent disease. The histological reporting of these features may be of value in predicting a group of women at high risk of recurrence of CIN after local treatment.