BACKGROUND In 2011, fee-for-service patients with both Medicare and Medicaid (dual eligible) sustained $319.5 billion in health care costs. OBJECTIVE To describe the emergency department (ED) use and hospital admissions of adult dual eligible patients aged under 65 years who used an urban safety net hospital. METHODS This was a retrospective database analysis of patients aged between 18 and 65 years with Medicare and Medicaid, who used an urban safety net academic health center between January 1, 2011, and December 31, 2011. We compared patients with and without behavioral health illness. The main outcome measures were hospital admission and ED use. Chi-square and Wilcoxon rank-sum tests were used for descriptive statistics on categorical and continuous variables, respectively. Greedy propensity score matching was used to control for confounding factors. Rate ratios (RR) and 95% confidence intervals (CI) were determined after matching and after adjusting for those variables that remained significantly different after matching. RESULTS In 2011, 10% of all fee-for-service dual eligible patients aged less than 65 years in Massachusetts were seen at Boston Medical Center. Data before propensity score matching showed significant differences in age, sex, race/ethnicity, marital status, education, employment, physical comorbidities, and Charlson Comorbidity Index score between patients with and without behavioral health illness. Analysis after propensity score matching found significant differences in sex, Hispanic race, and other education and employment status. Compared with patients without behavioral health illness, patients with behavioral health illness had a higher RR for hospital admissions (RR = 2.07; 95% CI = 1.81-2.38; P < 0.001) and ED use (RR = 1.61; 95% CI = 1.46-1.77; P < 0.001). Results were robust after adjusting for characteristics that remained statistically significantly different after propensity score matching. CONCLUSIONS Adult dual eligible patients aged less than 65 years with behavioral health illness in the Medicaid fee-for-service plan had significantly higher rates of hospital admission and ED use compared with dual eligible patients without behavioral health illness at the largest urban safety net medical center in New England. Safety net hospitals care for a large proportion of dual eligible patients with behavioral health illness. Further research is needed to elucidate the systems-related and patient-centered factors contributing to the utilization behaviors of this patient population. DISCLOSURES This research was funded in part by a National Research Service Award (T3HP10028-14-01). The authors have no conflicts of interests to disclose. Cancino had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design were contributed by Cancino, Jack, and Burgess, with assistance from Cremieux. Cancino and Cremieux took the lead in data collection, along with Jack and Burgess, and data interpretation was performed by Jarvis, Cummings, and Cooper, along with the other authors. The manuscript was written primarily by Cancino, along with Jack and Burgess, and revised primarily by Cancino, along with the other authors.