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We aimed to quantify whether vegetation level, measured by Normalized Difference Vegetation Index (NDVI) modifies associations between daily exposure to particulate matter (PM10, PM2.5) and hospitalization across 364 urban U.S. counties (2000-2013). Daily number of hospital admissions (≥65y, cardiovascular or respiratory diseases) from Medicare enrollees were used to assess risks as percent change in hospitalization related to 10µg/m3 increase in monitored daily particulate matter. We computed an absolute change in county-specific risks explained by difference in county-level NDVI. An interquartile range increase in NDVI corresponded to a 1.68% (95% CI: 0.43, 2.91) absolute decrease in cardiovascular hospitalization risk and 10.40% (95% CI: 7.34, 13.34) absolute decrease in acute myocardial infarction hospitalization risk associated with PM10. Reduction was generally larger for those 65-74 and 75-84y than those >85y. This study adds evidence for health benefits of green space for reducing the impacts of particulate matter on hospitalizations in the U.S.