Hollander MAG, Cole ES, Sabik LM, et al. - The Patient Protection and Affordable Care Act’s Medicaid expansion has led more than 17 million people to gain health insurance coverage. Researchers sought to examine heterogeneity within the Medicaid expansion population based on time-varying patterns of emergency department (ED) and ambulatory care use. In addition, they characterized diagnoses linked with ED and ambulatory care visits, to examine if certain diagnoses predominate in individual trajectories. Based on joint trajectories of ED and ambulatory care use, 6 distinct groups were identified among 601,877 expansion enrollees. The groups varied in mean ED use from 3.4 to 48.7 visits per 100 enrollees in the first month and between 2.8 and 44.0 visits per 100 enrollees in month 12. Findings thereby suggest substantial variation in rates of ED and ambulatory care use across empirically defined subgroups of Medicaid expansion enrollees. In addition, heterogeneity was observed among the diagnoses linked with these visits. They suggest a possible utility of this data-driven approach to target resources to encourage efficient use of ED services and support engagement with ambulatory care clinicians.