Method: This study is designed as an accompaniment to an already funded study by the DOD - a 12-week treatment trial with prazosin for patients with PTSD and AD. The current proposal will augment the findings from the treatment study and identify for which patients prazosin may be effective. It is important to note that this study is very different from the PTSD Coop study in that study EXCLUDES individuals with alcohol dependence. The DOD study and this companion study will provide answers relevant to alcohol use, alcohol relapse and drinking outcomes.