Data sources PubMed, Scopus and Web of Science databases. Study selection Cohort, case-control or retrospective studies involving more than 50 patients with biopsy proven diagnosis and absence of dysplasia at initial diagnosis of oral lichen planus (OLP) or oral lichenoid lesions (OLL) were considered. Data extraction and synthesis Two reviewers independently selected the studies and abstracted data. Study quality was assessed using the Newcastle-Ottawa Scale. A third reviewer arbitrated any disagreements. A random effects meta-analysis of malignant transformation rates was conducted. Results Twenty-one studies were included, 18 were retrospective and three prospective. In total there were 6559 patients; 6353 with OLP and 206 with OLL. The follow-up periods ranged from 18-300 months. Ninety-two cases of oral squamous cell carcinoma developed during the observation period. The overall transformation rate (TR) was 1.4%; 1.37% for OLP and 2.43% for OLL. Conclusions This systematic review confirms that both OLP and OLL, the latter with a slightly higher TR, may be considered potentially malignant disorders and suggest that erosive type, female gender and tongue site should be considered as risk factors for OLP transformation. Major efforts should be made to establish strict clinical and histological criteria to diagnose OLP and to perform sounder methodological observational studies.