This study, conducted from April to May 2004 in the metropolitan and surrounding areas of Seoul, Korea, was performed to show the relationship between indoor and outdoor levels of PM10 and PM2.5 concentrations in 14 residential houses. In addition, indoor/outdoor ratios of PM10, PM2.5 concentrations were calculated. The relationship between the PM10, PM2.5 concentrations and respiratory symptoms by self recording questionnaire of 14 houses was investigated. In conclusion, although the results of this study failed to establish the relationship between PM10, PM2.5 concentrations and respiratory symptoms among residents, the levels of indoor PM2.5 were significantly higher than those of outdoor levels. The indoor PM10, PM2.5 concentrations were increased by the amount of time spent of residents. Further research should be directed to establish the relationship between PM10, PM2.5 concentration and respiratory symptoms. Keywords：PM10, PM2.5, Concentration, Indoor, Outdoor, Respiratory symptom, I/O ratio