Psychiatric disorders in military members require substantial medical, administrative, and financial resources, and are among the leading causes of hospitalization and early discharge. We reviewed available data to better understand the incidence of bipolar I disorder among military personnel. Defense Medical Epidemiology Database inpatient data were used. Descriptive and comparative statistics were performed. From 1997-2006 there were 3,317 first hospitalizations for bipolar I disorder with a mean of 1.2 hospitalizations per case. The rate of first occurrence among this adult population was 0.24 per 1,000 person-years. The incidence increased over time for depressed and mixed episode types among both genders. High risk groups include women, younger individuals, and whites. This population provides insight into adult onset bipolar I disorder incidence and demographic patterns not available elsewhere and offers potential opportunities to improve its understanding.