The editorial quotes the statement by Robins and Guze that reliability is the “first test of validity for diagnosis” (3). But “first” refers to time sequence only, not to the word “foremost.” Validity, estimating the truth or falsity of scientific propositions, requires meeting three additional criteria. First, distinct boundaries must separate a disorder from other disorders. But the comorbidity of PTSD with generalized anxiety disorder, depression, and substance dependence (4) fails this requirement. Second, genetic aggregation should characterize family members of individuals diagnosed. Genetic association studies of PTSD have identified promising candidates such as the serotonin transporter gene (SLC6A4) (5). But robust confirmation requires extensive research efforts devoted to this gene and other suspects, requiring sample sizes possibly in the thousands. And third, the diagnosis must be substantiated by quantitative evidence (i.e., biological assays, brain imaging, or psychological tests). But pervasive contradictions and conflicting claims bedevil PTSD studies examining the morphology of the hippocampus and measures of substances such as catecholamines, cortisol, or glutamates.