Numerous epidemiologic studies have reported increases in the daily incidence of cardiovascular mortality and morbidity associated with increases in daily levels of particulate matter air pollution. We studied the association between the incidence of primary cardiac arrest and two daily measures of particulate matter using a case-crossover study of 362 cases of out-of-hospital cardiac arrest. All cases were attended by paramedics and had no history of clinically recognized heart disease or life-threatening comorbidities. We compared particulate matter levels at index times with particulate matter levels from referent days matched on day of week within strata defined by month and year. The estimated relative risk at a lag of 1 day for an interquartile range (IQR) change in nephelometry (0.51 x 10(-1) km(-1)) was 0.893 (95% CI = 0.779-1.024). The estimated relative risk at a lag of 1 day for an IQR change in PM10 (19.3 microg m(-3)) was 0.868 (95% CI = 0.744-1.012). Other lag periods gave similar results. We did not find evidence of confounding by carbon monoxide or sulfur dioxide. Analysis of effect modification by individual-level variables did not reveal any susceptible subgroups. These findings do no support an association between particulate matter and increased risk of primary cardiac arrest among persons without clinically recognized heart disease. The null results of this study may result from several factors, including the highly selected nature of this case series and the relatively low particulate matter levels in the Seattle metropolitan area.