One hundred twenty-four male Vietnam War veterans with chronic, severe posttraumatic stress disorder (PTSD) were randomly assigned to imagery rehearsal (n = 61) or a credible active comparison condition (n = 63) for the treatment of combat-related nightmares. There was pre-post change in overall sleep quality and PTSD symptoms for both groups, but not in nightmare frequency. Intent-to-treat analyses showed that veterans who received imagery rehearsal had not improved significantly more than veterans in the comparison condition for the primary outcomes (nightmare frequency and sleep quality), or for a number of secondary outcomes, including PTSD. Six sessions of imagery rehearsal delivered in group format did not produce substantive improvement in Vietnam War veterans with chronic, severe PTSD. Possible explanations for findings are discussed.