This study was performed to examine the relationship between particulate matter exposure and mortality in Seoul, Korea, during the year 2001. Particulate matter data were collected using an optical particle counter (OPC) and national monitoring stations in Seoul. The size-resolved aerosol number concentrations of particles 0.3-25 microm in diameter and mass concentrations of PM10 (particulate matter less than 10 microm in diameter) and PM2.5 (less than 2.5 microm in diameter) were measured. Meteorological data such as air temperature and relative humidity were provided by the Korea Meteorological Administration. Daily mortality was analyzed using a generalized additive Poisson model, with adjustment for the effects of seasonal trend, air temperature, humidity, and day of the week as confounders, in a nonparametric approach. We used S-Plus for all analyses. Model fitness, using loess smoothing, was based on stringent convergence criteria to minimize the default convergence criteria in the S-Plus generalized additive models module. The IQR (interquartile range) increase of fine particle (10.21 number/cm3 [the total number of particles per cubic centimeter]) and respiratory particle (10.38 number/cm3) number concentration were associated with a 5.73% (5.03%-6.45%) and a 5.82% (5.13%-6.53%) increase in respiratory disease-associated mortality, respectively. Mortality effects in the elderly (aged over 65 years) were increased by more than 0.51% to 2.59%, and the relative risks of respiratory-related and cardiovascular-related mortality were increased by 0.51% to 1.06% compared with all-cause mortality. These findings support the hypothesis that air pollution is harmful to sensitive subjects, such as the elderly, and has a greater effect on respiratory- and cardiovascular-related mortality than all-cause mortality. However, our results using OPC data did not support the hypothesis that PM2.5 would have more adverse health effects than PM10 in number concentration but not in mass concentration.