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Objective: Obsessive-compulsive disorder (OCD) is a clinically heterogeneous condition. This heterogeneity can reduce the power and obscure the findings from natural history studies to genome scans, neuroimaging, and clinical trials. The authors review the evidence supporting a multidimensional model of OCD. Method: Computerized and manual literature searches were performed to identify factor-analytic studies of obsessive-compulsive symptoms before data from disciplines that bear on the potential usefulness of these dimensions were considered. Selection criteria included the novelty and importance of studies and their relevance to outcomes of interest to well-informed mental health professionals. Results: Twelve factor-analytic studies involving more than 2,000 patients were identified that consistently extracted at least four symptom dimensions: symmetry/ordering, hoarding, contamination/ cleaning, and obsessions/checking. These dimensions were associated with distinct patterns of comorbidity,genetic transmission, neural substrates, and treatment response. The evidence supporting the hoarding dimension is particularly robust. Conclusions: The complex clinical presentation of OCO can be summarized with a few consistent, temporally stable symptom dimensions. These can be understood as a spectrum of potentially overlapping syndromes that may 1) coexist in any patient, 2) be continuous with normal obsessive-compulsive phenomena, and 3) extend beyond the traditional nosological boundaries of OCD. Although the dimensional structure of obsessive- compulsive symptoms is imperfect, this quantitative approach to phenotypic traits has the potential to advance our understanding of OCD and may aid in the identification of more robust endopheno-types. The need for a dimensional rating scale and suggestions for future research aimed at reducing the burden of this disorder are discussed. [ABSTRACT FROM AUTHOR]