Recently, we reported the preliminary results of a prospective study on the possible premalignant character of oral lichen planus (OLP) and oral lichenoid lesions (OLL). Based on these data it was concluded that there was some but no convincing support for the hypothesis that patients with OLL have an increased risk of development of oral cancer, but not so in patients with OLP. In the present treatise the results of prolonged follow-up of this cohort of patients have been described. A study group of 192 patients, 67 patients diagnosed with OLP and 125 patients with OLL, according to revised World Health Organization diagnostic criteria, was followed for periods ranging from 7.6 to 96.9 months (mean, 55.9 months). The expected number of patients with oral cancer in the group of patients with OLP and in the group of patients with OLL was estimated by comparing the number of patients, their ages, sex, and the length of follow-up to annual incidence rates of oral cancer for the general population in The Netherlands. The binomial test was used to determine whether the observed number of cases of cancer in the OLP group and the OLL group exceeded the expected numbers. Four out of 192 patients, two men and two women, developed a squamous cell carcinoma of the oral mucosa during follow-up. All malignant transformations occurred in the OLL group. The malignant transformation of the OLL group, based on a mean follow-up of 53.8 months, was calculated at 0.71% per year. A comparison of the expected against actual figures for the development of carcinomas revealed no increase in patients with OLP and a 142-fold increase in patients with OLL, the latter being statistically significant, with a p-value of 0.044. The present data give support to the hypothesis that patients with OLL have an increased risk of development of oral cancer. There seems to be no increased risk in patients with OLP. In view of our results we advise to monitor only the subgroup of OLL patients twice a year for early detection of possible malignant transformation.