Background: Several studies have identified discrete symptom dimensions in obsessive–compulsive disorder (OCD), derived from factor analyses of the individual items or symptom categories of the Yale–Brown Obsessive–Compulsive Scale Symptom Checklist (YBOCS-SC). This study aims to extend previous work on the relationship between obsessions and compulsions by specifically including mental compulsions and reassurance-seeking. Because these compulsions have traditionally been omitted from prior factor analytic studies, their association to what have been called “pure obsessions” may have been overlooked. Method: Participants (N=201) were recruited from two multi-site randomized clinical treatment trials for OCD. The YBOCS-SC was used to assess OCD symptoms, as it includes a comprehensive list of obsessions and compulsions, arranged by content category. Each category was given a score based on whether symptoms were present and if the symptom was a primary target of clinical concern, and a factor analysis was conducted. Mental compulsions and reassurance-seeking were considered separate categories for the analysis. Results: Using an orthogonal geomin rotation of 16 YBOCS-SC categories/items, we found a five-factor solution that explained 67% of the total variance. Inspection of items that composed each factor suggests five familiar constructs, with mental compulsions and reassurance-seeking included with sexual, aggressive, and religious obsessions (unacceptable/taboo thoughts). Conclusions: This study suggests that the concept of the “pure obsessional” (e.g., patients with unacceptable/taboo thoughts yet no compulsions) may be a misnomer, as these obsessions were factorially associated with mental compulsions and reassurance-seeking in these samples. These findings may have implications for DSM-5 diagnostic criteria. Depression and Anxiety, 2011. © 2011 Wiley-Liss, Inc.