The radon exhalation rates from surfaces of different types of lightweight concrete used in the building industry in Hong Kong have been studied using standardised activated charcoal canisters and y spectroscopy. It is found that all the lightweight concretes investigated have considerably smaller radon exhalation rates than those from ordinary concrete. Considering a concrete room of a typical size for Hong Kong, the possible reduction in the indoor radon concentrations has been calculated to be greater than 15 Bq.m -3 when lightweight concrete is used instead of ordinary concrete for the non-construction walls. The average indoor radon concentration in Hong Kong is about 45 Bq.m -3 . Therefore, a simple and economical way to reduce the indoor radon concentrations and the corresponding radiation dose from radon has been demonstrated. This technique applies to future buildings.