The frequency of symptoms of posttraumatic stress disorder within each of three categories—reexperiencing, avoidance or numbing, and physiological arousal—was examined in 116 Vietnam combat veterans with a diagnosis of PTSD. The prevalence of all PTSD symptoms was greater than 50 percent except for flashbacks, psychogenic amnesia, and sense of foreshortened future. Comorbidity in a subgroup of 48 patients was assessed using operational criteria for DSM-III-R mental disorders. Mood disorders, psychoactive substance abuse disorders, and other anxiety disorders frequently co-occurred with PTSD, but psychotic disorders were uncommon. These findings provide empirical validation of the DSM-III-R diagnostic criteria for PTSD.