Underserved populations often utilize the emergency room in place of primary care, particularly for short term behavioral health services. This study examined emergency department (ED) utilization rates for rurality, insurance, sex, and ethnicity in a large sample of adult patients in Hawai'i hospitals from 2000-2010 with a diagnosis of post-traumatic stress disorder (PTSD), mood disorder, or other anxiety disorder. Findings showed a higher rate of use by rural and suburban residents with a diagnosis of PTSD or other anxiety than by urban residents. Utilization of EDs by African Americans and Other Pacific Islanders with PTSD was proportionally higher than for those with mood disorders or other anxiety disorders. Military ED visits were also proportionally higher for individuals with PTSD than for those with mood or other anxiety disorders. Limited economic resources and increasingly costly mental health disorders such as PTSD highlight the importance of better understanding the needs for behavioral health services for underserved populations.