It is clear from the existing data that PTSD often occurs in the context of other major psychological conditions. Evidence to support this comes from clinical studies, epidemiological studies, and studies of PTSD among substance abusers. Clearly, probably several different subgroups of PTSD patients exist including those who had psychological or behavioral problems before exposure to traumatic events (e.g., substance abuse), those who developed other problems concurrent with exposure to the traumatic events, and those who developed problems secondary to the development of PTSD, perhaps in efforts to cope with the intensely debilitating symptoms of PTSD. With this knowledge, research on PTSD must begin to contend with the comorbidity issue in systematic ways. The use of comparison groups that are carefully selected is one key way in which conclusions about PTSD can be most conservatively drawn. The use of statistical procedures to control for difference in levels of comorbidity is another responsible way in which to approach the problem. Finally, efforts to employ global measures of functioning such as the Global Assessment of Functioning to equate subjects within a study on minimally this characteristic may be the most economical method for trying to rule out the role of comorbidity and severity of condition in conclusions drawn in research studies. All these solutions presuppose the careful measurement of comorbidity in studies of PTSD, a recommendation that requires serious consideration for researchers operating in this field.