Heidnik served in the Army for 13 months. During basic training, Heidnik's drill sergeant graded him as "excellent." Following basic training, he applied for several specialist positions, including the military police, but was rejected. He was sent to San Antonio, Texas to be trained as a medic, and did well through medical training. However, Heidnik did not stay in San Antonio very long, and was transferred to the 46th Army Surgical Hospital in Landstuhl, West Germany. Within weeks of his new posting in Germany, he earned his GED. In August 1962, Heidnik began complaining of severe headaches, dizziness, blurred vision, and nausea. A hospital neurologist diagnosed Heidnik with gastroenteritis, and noted that Heidnik also displayed symptoms of mental illness, for which he was prescribed trifluoperazine (Stelazine). In October 1962, Heidnik was transferred to a military hospital in Philadelphia, Pennsylvania, where he was diagnosed with schizoid personality disorder, and consequently honorably discharged from military service.