Understanding readmissions within the Military Health System (MHS) provides important insights to better understand and improve health outcomes for military personnel and their families. We assessed all-cause seven- and thirty-day readmission rates in military treatment facilities by treatment service for patients ages 18-64 for fiscal years 2011-18 using inpatient data from the MHS and the private sector. We compared unplanned readmission rates for the obstetric, medical, and surgical product lines. Readmission rates differed by product line. Seven-day readmission rates ranged from 1.5percent to 3.3percent by product line, and thirty-day rates ranged from 3.2percent to 8.8percent. The obstetric line had the greatest number of readmissions (391,463) but the lowest seven-day readmission rate (1.5percent). Readmission rates were lower for the military population than for people in other insurance groups, but military readiness is disrupted by unplanned readmissions. Product-line differences in readmission rates in the MHS suggest opportunities for improvement.