There is evidence that the major anxiety and depressive disorders could reflect a single underlying internalization factor. For a group of 1,031 clinic-referred children, the study examined support for this factor, and used the two-parameter logistic model to examine the item response theory properties of the disorders in this factor. For the set of anxiety and depressive disorders, confirmatory factor analysis supported a one-factor model. The two-parameter logistic model analysis indicated that all the internalizing disorders in this factor were strong discriminators of the internalizing dimension. Also, they measured more of the internalizing dimension and with more precision in the upper half of the trait continuum. There was also support for the convergent validity of the internalizing dimension, in that it had large-to-medium effect size correlations with internalizing scores of other measures. The implications of the findings for clinical practice and clinical classification are discussed.