BACKGROUND: Depression is a clinically heterogeneous disorder thought to result from multiple genes interacting with environmental and developmental components. A dimensional rather than a categorical approach to depressive phenotype definition may be more useful for identification of susceptibility genes. OBJECTIVES: To perform an exploratory factor analysis on a range of depressive and anxiety symptoms in a large, well-defined sample of depressed siblings, as well as a confirmatory factor analysis in a separate large group of unrelated depressed subjects, and to analyze correlations of identified symptom dimensions between depressed siblings. DESIGN: Subjects (N = 1034), including 475 sibling pairs, with a history of at least 2 depressive episodes were recruited from the Depression Network Study, a large-scale multicenter collection of families affected by recurrent unipolar depression. Subjects were interviewed using the Schedules for Clinical Assessment in Neuropsychiatry (SCAN) and diagnosed according to the DSM-IV and the International Classification of Diseases, 10th Revision, using a computerized scoring program (CATEGO5). Factor analysis was carried out on 26 depression symptom items, including 4 anxiety screening items. Confirmatory factor analysis was performed on an independent sample of 485 depressed individuals. RESULTS: Four interpretable factors were identified: (1) mood symptoms and psychomotor retardation; (2) anxiety; (3) psychomotor agitation, guilt, and suicidality; and (4) appetite gain and hypersomnia. For each symptom group, a quantitative scale was constructed, and correlations between siblings were calculated. There was a moderate degree of sibling homotypia for some depressive symptoms, and factors 1, 2, and 3 showed significant positive familial correlation (0.145 [P =.001], 0.335 [P<.001], and 0.362 [P<.001], respectively). CONCLUSIONS: This is the first study of large, well-defined samples of depressed subjects in whom symptom dimensions have been derived and then confirmed using independent material. The significant correlations between siblings for 3 of the dimensions suggest substantial familial, perhaps genetic, etiologies.