Military service requires individuals who are free from ill-health and who are physically and mentally robust, and throughout history those who have become unfit for service have been discharged on medical grounds. This paper uses published historical records to examine trends in the rate of discharge for six key conditions for the period 1861-1998 and to draw conclusions. In the late 19th and early 20th centuries, communicable diseases such as tuberculosis and syphilis were a major problem together with ill-defined conditions such as "disordered action of the heart". By the mid-20th century, psychiatric and gastrointestinal disorders had become the principal causes of medical discharge, whereas in the late 20th century, the majority of discharges resulted from injuries and musculoskeletal disorders. The pattern of conditions leading to medical discharge from the Army tends to mirror conditions which are prevalent at that time in the civilian population.