Indoor radon/thoron concentration has been measured in Aizawl district, Mizoram, India, which has the highest lung cancer incidence rates among males and females in India. Simultaneously, radon flux emanated from the surrounding soil of the dwellings was observed in selected places. The annual average value of concentration of radon(thoron) of Aizawl district is 48.8(22.65) Bq m−3 with a geometric standard deviation of 1.25(1.58). Measured radon flux from the soil has an average value of 22.6 mBq m−2 s−1. These results were found to be much below the harmful effect or action level as indicated by the World Health Organisation. On the other hand, food habit and high-level consumption of tobacco and its products in the district have been found to increase the risk of lung cancer incidence in the district.