Exposure to particulate matter (PM) is one of the important risk factors for morbidity and mortality. Although PM concentrations have been assessed using air quality monitoring stations or modelling, few studies have measured indoor PM in large-scale birth cohorts. The Japan Environment and Children's Study (JECS) measured indoor and outdoor air quality in approximately 5000 households when the participating children were aged 1.5 and 3 years. PM was collected using portable pumps for 7 days (total of 24 h), inside and outside each home. Prediction models for indoor PM concentrations were built using data collected at age 1.5 years and post-validated against data collected at age 3 years. Median indoor/outdoor PM2.5 and PM10-2.5 concentrations at age 1.5 years [3 years] were 12.9/12.7 [12.5/11.3] mu g/m(3) and 5.0/6.3 [5.1/6.1] mu g/m(3), respectively. Random forest regression analysis found that the major predictors of indoor PM2.5 were indoor PM10-2.5, outdoor PM2.5, indoor smoking, observable smoke and indoor/outdoor temperature. Indoor PM2.5, outdoor PM10-2.5, indoor humidity and opening room windows were important predictors of indoor PM10-2.5 concentrations. Indoor benzene, acetaldehyde, ozone and nitrogen dioxide concentrations were also found to predict indoor PM2.5 and PM10-2.5 concentrations, possibly due to the formation of secondary organic aerosols. These findings demonstrate the importance of reducing outdoor PM concentrations, avoiding indoor smoking, using air cleaner in applicable and diminishing sources of VOCs that could form secondary organic aerosols, and the resulting models can be used to predict indoor PM concentrations for the rest of the JECS cohort.