Objective To understand the indoor air pollution in Beijing. Methods Based on the data from epi-demiological investigation and air pollution monitoring, the investigation was carried out by choosing 3 districts in Beijing including Dongcheng, Shijingshan and Haidian during Jan-Mar in 2000. 70 houses were selected in each district randomly by cluster sampling, the indoor concentrations of PM10, PM2.5 and SO2 in bedroom and kitchen were monitored. The data on concentrations of PM10, PM2.5 and SO2, and the environmental conditions of each sampling place were all recorded in the unified questionaires. Results The total average concentrations of pollutants in indoor air of 3 districts were 0. 555 mg/m3 for PM10 and 0. 512 mg/m3 for PM2.5 respectively. The total average concentration of 0.052 mg/ m3 for SO2 was below the ralated National Stardard. The concentrations of PM10 and PM2.5 in the bedrooms and kitchens in each district all exceeded the related national standards, and revealed no significant differences between bedrooms and kitchens. The SO2 levels in kitchens in Dongcheng District and Haidian District were significantly higher than those in bedrooms(P