Oral submucous fibrosis (OSMF) is a crippling disorder which is confined almost exclusively to the Indian subcontinent. Despite its association with a significantly increased risk of cancer, the etiology is still not clear. An epidemiological assessment showed 0.4% prevalence for OSMF in Kerala, South India, which is among the highest recorded. Recently the National Tumour Registry in Trivandrum reported the highest recorded site-specific incidence rate for oral cancer (ICD 140-145) in this area. The coastal belt of the Trivandrum and Quilon districts of Kerala has a very high natural radioactivity (over 1500 mR (387 microC) per year); about 500 mR (129 microC) per year is considered to be the maximum permissible dose for populations in general. An epidemiological survey in this area and in a comparable population (without exposure to high background radiation) as a control showed that the percentage prevalence of OSMF in the study area was 0.27 and in the control area 0.32. It appears highly improbable that the OSMF in the study area was induced by high background radiation.