residence using five-digit ZIP code information provided at enrollment according to the ZIP code boundaries (STF3B) of the 1980 United States Census (13). Three different ecologic measures of PM2.5 were used as indicators of historical PM2.5 exposure. Average ambient PM2.5 concentrations for the 4-year period (1979–1983) encompassing the year of enrollment were obtained for 131,864 participants residing in 61 MSAs from the Inhalable Particle Monitoring Network, as compiled by the Health Effects Institute reanalysis team (14). Average ambient PM2.5 concentrations were also available in 1999 and in the first three quarters of 2000 for 177,752 participants residing in 117 MSAs from the Aerometric Information Retrieval System, implemented in response to the 1997 United States Environmental Protection Agency PM2.5 standard. Quarterly mean PM2.5 concentrations were determined by site and MSA and averaged when there were at least 50% of sixth-day samples and at least 45 total sampling days in one of the two corresponding quarters. Because there was no systematic monitoring of PM2.5 in the United States in the period spanning the early 1980s to the late 1990s, a third measure representing the average of PM2.5 concentrations in the two time periods (1979–1983 and 1999–2000) was also constructed for 120,917 participants in 53 MSAs. These indicators of ambient PM2.5 concentrations have been extensively examined in relation to mortality health effects in the CPS-II (6, 14, 15).