STUDY OBJECTIVE Cost concerns may inhibit emergency medical services (EMS) use. Novel tax-based and subscription prepayment programs indemnify patients against the cost of EMS treatment and transport. We determine whether the presence of (or enrollment in) prepayment plans increase EMS use among patients with acute chest discomfort, particularly those residing in low-income areas, those lacking private insurance, or both. METHODS This study uses a subset of baseline data from the REACT trial, a multicenter, randomized controlled community trial designed, in part, to increase EMS use. The sample includes 860 consecutive noninstitutionalized patients (>30 years old) presenting with nontraumatic chest discomfort to hospital emergency departments in 4 Oregon/Washington communities. The association between prepayment systems and EMS use was analyzed using multivariable logistic regression. RESULTS Overall EMS use was 52% (n=445). Among EMS users, 338 (75%) were subsequently admitted to the hospital and 110 (25%) were released from the ED. Prepayment was not associated with increased EMS use in the overall patient sample. However, patients residing in low-income census block groups (median annual income <$30,000) were 2.6 times (95% confidence interval [CI] 1.4 to 4.8) more likely to use EMS when a prepayment system was available than when no system was present. No association was noted among higher-income block group residents. Among low-income block group residents lacking private insurance, prepayment systems were associated with 3.8 times (95% CI 1.2 to 13.4) greater EMS usage. CONCLUSION Economic considerations may affect EMS system utilization among underinsured and low-income patients experiencing a cardiac event. Prepayment systems may increase EMS utilization among these groups.