There has been considerable controversy as to whether oral lichen planus (LP) has a premalignant potential. This study retrospectively examined the records of 241 British patients with histologically confirmed LP seen during the 10-year period 1982-92. Nine patients (3.7%) were known to have developed well-differentiated invasive carcinoma or carcinoma in situ in an area of LP. Most carcinomas at presentation were in areas of atrophic and/or erosive LP, 6 were in patients older than 65 years, and 6 were on the tongue. The results support a small but clinically important premalignant potential for LP.