Recent research suggests that some cases of cardiovascular mortality may be related to carbon monoxide (CO) air pollution. Clinically based studies indicate the adverse effects of CO on the cardiopulmonary system. However, little attention has been paid to the question of hospital admissions for cardiovascular illness caused by ambient CO levels. The present study assesses the association between hospital admissions for cardiovascular system illnesses and the ambient levels of CO in the Reno-Sparks, NV, area over a 6-yr period (1989-1994). Daily admissions to all three hospitals in the region and daily ambient concentrations of CO, monitored at five sites, were included. There were 32,705 total cardiovascular (CV) admissions, including 13,108 with the diagnosis of ischemic heart disease (IHD) during the study period. The average daily 1-h maximum level of CO was 3.09 ppm. After adjusting for day-of-the-week and seasonal effects and controlling for the effects of autocorrelation errors, both weighted least squares (WLS) and autoregressive integrated moving average (ARIMA) methods showed consistently positive relationships between the ambient CO level and different groups of cardiovascular admissions, although the male gender and age older than 60 groups tended to be most affected. Data suggest a positive correlation between ambient CO levels and hospital admissions for CV diseases.