Oral sub mucous fibrosis is an insidious chronic disease affecting any part of the oral cavity. Worldwide estimates of oral sub mucous fibrosis indicate that 2.5 million people are affected, with most cases concentrated in the Indian subcontinent, especially eastern and southern India. Oral sub mucous fibrosis has a propensity for malignant transformation. The association of betel quid chewing, oral sub mucous fibrosis, and oral squamous cell carcinoma is quite profound, especially in Taiwan and the Indian subcontinent where up to 80% of oral squamous cell carcinoma is associated with the habit. Epidemiological studies have shown that the rate of malignant transformation ranges from 3 to 19%. © 2014 Elsevier B.V., All rights reserved.; MEDLINE® is the source for the MeSH terms of this document.