The authors hypothesized that anxiety and unipolar mood disorders are often comorbid because each disorder indicates a broad, higher order factor. In a clinical subsample of the nationally representative National Comorbidity Survey participants (N = 251), a one-factor model fit the correlations among 7 dichotomous anxiety and unipolar mood diagnoses. Following the lead provided by literature on the structure of emotional and behavioral problems in children, we labeled this factor internalizing. Item response theory was used to explore how each diagnosis mapped onto the internalizing factor. The test information function derived from the 7 diagnoses suggested that they measure primarily the higher end of the factor. In addition, very high scores on internalizing (meeting criteria for 6-7 disorders) were associated with increased social costs, a phenomenon not well captured by the "comorbidity" concept. The results underscore the need to develop clinical assessment instruments that span the full range of the internalizing factor and measure both the shared and distinctive features of anxiety and unipolar mood disorders in a graded, continuous fashion.