Introduction: Behavioral health conditions are a significant concern for the U.S. military and the Military Health System (MHS) because of decreased military readiness and increased health care utilization. Although MHS beneficiaries receive direct care in military treatment facilities, a disproportionate majority of behavioral health treatment is purchased care received in civilian facilities. Yet, limited evidence exists about purchased behavioral health care received by MHS beneficiaries. This longitudinal study (1) estimated the prevalence of purchased behavioral health care and (2) identified patient and visit characteristics predicting receipt of purchased behavioral health care in acute care facilities from 2000 to 2014., Materials and Methods: Medical claims with Major Diagnostic Code 19 (mental disorders/diseases) or 20 (alcohol/drug disorders) as primary diagnoses and TRICARE as the primary/secondary payer were analyzed for MHS beneficiaries (n = 17,943) receiving behavioral health care in civilian acute care facilities from January 1, 2000, to December 31, 2014. The primary dependent variable, receipt of purchased behavioral health care, was modeled for select mental health and substance use disorders from 2000 to 2014 using generalized estimating equations. Patient characteristics included time, age, sex, and race/ethnicity. Visit types included inpatient hospitalization and emergency department (ED). Time was measured in days and visits were assumed to be correlated over time. Behavioral health care was described by both frequency of patients and visit type. The University of South Carolina Institutional Review Board approved this study., Results: From 2000 to 2014, purchased care visits increased significantly for post-traumatic stress disorder, adjustment, anxiety, mood, bipolar, tobacco use, opioid/combination opioid dependence, nondependent cocaine abuse, psychosocial problems, and suicidal ideation among MHS beneficiaries. The majority of care was received for mental health disorders (78.8%) and care was most often received in EDs (56%). Most commonly treated diagnoses included mood, tobacco use, and alcohol use disorders. ED visits were associated with being treated for anxiety (excluding post-traumatic stress disorder; Adjusted odds ratio [AOR]: 9.14 [95% confidence interval (CI): 8.26, 10.12]), alcohol use disorders (AOR = 1.67 [95% CI: 1.53, 1.83]), tobacco use (AOR = 1.16 [95% CI: 1.06, 1.26]), nondependent cocaine abuse (AOR = 5.47 [95% CI: 3.28, 9.12]), nondependent mixed/unspecified drug abuse (AOR = 7.30 [95% CI: 5.11, 10.44]), and psychosis (AOR = 1.38 [95% CI: 1.20, 1.58]). Compared with adults age 60 yr and older, adolescents (ages 12-17 yr), and adults under age 60 yr were more likely to be treated for suicidal ideation, adjustment, mood, bipolar, post-traumatic stress disorder, nondependent cocaine, and mixed/unspecified drug abuse. Adults under age 60 yr also had increased odds of being treated for tobacco use disorders, alcohol use disorders, and opioid/combination opioid dependence compared with adults age 60 yr and older., Conclusions: Over the past 15 yr, purchased behavioral health care received by MHS beneficiaries in acute care facilities increased significantly. MHS beneficiaries received the majority of purchased behavioral health care for mental health disorders and were treated most often in the ED. Receiving behavioral health care in civilian EDs raises questions about access to outpatient behavioral health care and patient-centered care coordination between civilian and military facilities. Given the influx of new Veterans Health Administration users from the MHS, findings have implications for military, veteran, and civilian facilities providing behavioral health care to military and veteran populations.