The authors investigated measurement properties of the "Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition," generalized anxiety disorder (GAD) criteria in the National Comorbidity Survey and the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD). The two studies used different widely used instruments. There were significant (p less than 0.001) differences in measurement of GAD due to age, study, and age-study interaction on item thresholds and factor loadings of GAD, especially when different stem-probe structures of interviews were taken into account. Item thresholds were estimated to differ by as much as -0.74 as a function of age and 0.40 as a function of study. Despite these differences, factor scores derived from symptom criteria strongly predicted categorical diagnostic outcomes based on symptom count. It is concluded that interview structure, especially the stem-probe format of structured interviews, and wording had significant effects on study findings; that future studies in psychiatric epidemiology should use common structured interviews as much as possible; and that factor scores can be used in conjunction with sum scores as cut points to retain the advantages of both dimensional and categorical classification. (Contains 12 tables.)