This study examines the consistency of the association between particulate matter less than 10 μm in aerodynamic diameter (PM10), carbon monoxide (CO), and hospital admissions for cardiovascular disease across eight urban counties with different levels of correlation between pollutants and between the pollutants and weather. I fit Poisson regression models controlling for day-of-week effects and using nonparametric smoothing to control for season and weather. Daily variation in PM10 was associated with hospital admissions for heart disease in the elderly (2.48% increase, for an interquartile range increase in exposure; 95% confidence limits = 1.82%, 3.15%). Daily variation in CO was also associated with admissions (2.79% increase, for an interquartile range (1.75 parts per million) increase in exposure; 95% confidence limits = 1.89%, 3.68%). These associations held in both humid and dry locations and were independent of the correlation between the pollutants and weather or other pollutants. The effect size estimates for PM10 and CO likewise showed no association with their correlation in each location, as one would expect if one pollutant were merely a proxy for the other. The PM10 effects are consistent with recent animal data showing cardiovascular effects and with preliminary human data showing effects on heart rate variability and plasma viscosity. The CO results are consistent with literature on angina exacerbations. (Epidemiology 1999;10:17–22)