Changes in immune status have been suggested as a possible biologic mechanism by which particulate matter (PM) air pollution could lead to adverse health effects. The authors studied associations between ambient PM₂.₅ and immune status among 115 postmenopausal, overweight women in the greater Seattle, Washington, area. The authors evaluated 3-day, 30-day, and 60-day average PM₂.₅ values in relation to inflammation markers (C-reactive protein, serum amyloid A, interleukin-6) and functional assays of cellular immunity (natural killer cell cytotoxicity, T-lymphocyte proliferation) at 3 time points for each woman during 1 year. Three-day averaged PM₂.₅ was inversely associated with anti-CD3-stimulated lymphocyte proliferation. There were no notable associations between the inflammation markers and PM₂.₅. If additional studies confirm our findings, then future health effect assessments for PM₂.₅ should consider changes in cellular immunity as an endpoint that may lead to overt clinical disease.