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Populations susceptible to the effects of particulate matter have begun to be characterized, but the independent contributions of specific factors have not been explored.We used a case-crossover study to examine PM10-associated mortality risk during 1988–1991 among 65,180 elderly residents of Cook County, Illinois, who had a history of hospitalization for heart or lung disease. We assessed how the effect was independently modified by specific diagnoses and personal characteristics.We found a 1.14% (95% confidence interval [CI] = 0.44% to 1.85%) increased risk of death per 10 μg/m3 increase in ambient PM10 concentration. Persons with heart or lung disease—but no specific diagnosis of myocardial infarction, diabetes, congestive heart failure, chronic obstructive pulmonary disorder, or conduction disorders—were at 0.74% (−0.29% to 1.79%) increased risk. Persons with a history of myocardial infarction had a 2.7-fold higher risk (CI = -2.1 to 7.4). Those with diabetes carried a 2.0-fold higher risk (CI = -1.5 to 5.5). Risk appeared to decrease with age among elderly men and increase with age among elderly women, but the estimated 3-way interaction was not precise enough to exclude the null. We found no indication that susceptibility varied by group-level socioeconomic measures.Among a frail population, individuals diagnosed with myocardial infarction or diabetes were at greatest risk of death associated with high concentrations of PM10. These results suggest that their susceptibility may derive from prior vascular damage to the heart.