Recent studies showed that air pollution is a risk factor for hospitalization for myocardial infarction (MI). However, there is limited evidence to suggest which subpopulations are at higher risk for MI arising from air pollution. This study was undertaken to examine the modifying effects of specific secondary cardiovascular diagnosis (including hypertension, diabetes, congestive heart failure, and arrhythmias) on the relationship between hospital admissions for MI and exposure to ambient air pollutants. Hospital admissions for MI and ambient air pollution data for Taipei were obtained for the period 1999–2009. The relative risk of hospital admissions for MI was estimated using a case-crossover approach. None of the secondary diagnosis examined showed significant evidence of effect modification. It would appear that the correlation between air pollutant exposure and MI occurrence is not affected by predisposing factors present in other cardiovascular diseases.