The goals of this study were to examine relationships among symptom categories in obsessive-compulsive disorder (OCD), to establish OCD symptom dimensions by factor- and cluster-analytic analyses, and to explore associations between OCD symptom dimensions and comorbid neuropsychiatric conditions. A total of 317 OCD participants underwent a systematic diagnostic interview using the Structured Clinical Interview for DSM-IV. OCD symptoms assessed by the Yale-Brown Obsessive-Compulsive Scale Symptom Checklist (N=169) and by the Thoughts and Behaviors Inventory (N=275) were subjected to factor and cluster analyses. An identical four-factor solution emerged in two different data sets from overlapping samples, in agreement with most smaller factor-analytic studies employing the YBOCS checklist alone. The cluster analysis confirmed the four-factor solution and provided additional information on the similarity among OCD symptom categories at five different levels. OCD symptom dimensions showed specific relationships to comorbid psychiatric disorders: Factor I (aggressive, sexual, religious and somatic obsessions, and checking compulsions) was broadly associated with comorbid anxiety disorders and depression; Factor II (obsessions of symmetry, and repeating, counting and ordering/arranging compulsions) with bipolar disorders and panic disorder/agoraphobia; and Factor III (contamination obsessions and cleaning compulsions) with eating disorders. Factors I and II were associated with early onset OCD. This study encourages the use of cluster analyses as a supplementary method to factor analyses to establish psychiatric symptom dimensions. The frequent co-occurrence of OCD with other psychiatric disorders and the relatively specific association patterns between OCD symptom dimensions and comorbid disorders support the importance of OCD subtyping for treatment, genetic, and other research studies of this heterogeneous disorder.