This study assessed the contribution of baseline psychological symptoms, combat exposure, and unit support in the etiology of posttraumatic stress disorder (PTSD), and psychological distress. From 2004-2006, 67% of a random sample of 2,820 participants who had been assessed for psychological symptoms in 2002 were reassessed. Baseline psychological symptoms, combat exposure, and unit support factors were associated with the outcomes and the effect sizes for combat exposure were marked for PTSD symptoms. Adjustment for baseline psychological symptoms did not modify the pattern of association of group cohesion and combat exposures. The authors concluded that combat exposure and group cohesion have an effect on mental health outcomes independent of previous mental health status, which explains why screening prior to deployment is ineffective.