The present report describes malignant development in oral lichen planus (OLP) among 611 patients (409 F, 202 M) followed for periods from 1-26 years (mean: 7.5). During follow-up, 9 patients (1.5%), 8 women (1.9%) and 1 man (0.5%) developed oral squamous cell carcinomas (SCC) in areas of lichen planus lesions. The age of the patients at diagnosis of carcinoma ranged from 56-79 years (mean: 70.4) and the length of follow-up before malignant development ranged from 4.9-24 years (mean: 10.1). The estimated number expected to develop oral cancer in a sample of the general Danish population of similar size, age distribution and follow-up was 0.18 (0.11 F, 0.07 M) i.e., OLP cases showed a 50-fold increase (F = 70- M = 14-fold). The observed number of cancer cases was significantly higher than the estimated number (p less than 0.00001). Therefore, oral lichen planus fulfils the WHO criterion of a premalignant condition.