OBJECTIVE: To assess the potential malignant risk of vulval premalignant conditions, in particular, to investigate whether there is a difference in the cancer risk between women with the 2 types of vulval intraepithelial neoplasia (VIN). METHODS: All vulval biopsy specimens taken for any reason in a single center for a 5-year period were identified. The histologic reports of 1309 biopsy specimens from 802 women were reviewed, and all pathologic conditions present were recorded for each woman. Reports of patients with biopsy specimens containing usual-type VIN, differentiated-type VIN, lichen sclerosus, and squamous hyperplasia were selected and analyzed for the presence of metachronous or subsequent carcinoma to give a proportional risk for each condition. RESULTS: Five hundred eighty women were identified with premalignant vulval conditions: 171 had usual-type VIN, 70 had differentiated-type VIN, 191 had lichen sclerosus, 145 had squamous hyperplasia, and 3 had other conditions not included in this analysis. Within these groups, the numbers of women with prior, synchronous, or subsequent vulval squamous cell carcinoma were 44 (25.7%), 60 (85.7%), 53 (27.7%), and 53 (31.7%), respectively (P = 0.000). CONCLUSIONS: Differentiated-type VIN is significantly more associated with vulval squamous cell carcinoma than usual-type VIN.