This paper contributes to research on quality drivers in healthcare settings by examining the relationships between patient volume, teaching mission, and process quality in US hospitals. To develop a model that accurately assesses the impact of patient volume and teaching status on quality, we draw on three related research streams pertaining to the volume–quality relationship, the comparative quality of care in teaching and non-teaching hospitals, and quality drivers in service institutions. We propose the impact of patient volume on process quality varies across hospitals with different teaching intensities. The test of this proposition uses a large data set that measures process quality for treatments for heart attacks and heart failures in all major US hospitals. Our results suggest that, as hospital teaching intensity increases, greater patient volume is associated with decreased process quality. Never before was such a relationship uncovered. This initial finding has important practical implications. First, the regionalization policy of hospitals should be re-evaluated in light of their teaching function. Second, the root causes for the lower quality scores of large, high resident-to-bed ratio teaching hospitals, compared with smaller versions, must be found.