Background: Obsessive-compulsive disorder (OCD) is a chronic and debilitating anxiety disorder associated with significant impairment in quality of life and functioning. Research examining the differences in clinical correlates and treatment response associated with different obsessions in OCD has yielded important findings underscoring the heterogeneous nature of this disorder. To date, most of this research has focused on differences associated with primary obsessions, and little attention has been paid to the clinical utility of studying how compulsive symptoms affect clinical course. Virtually no systematic research has explored the clinical characteristics of one understudied symptom presentation, mental rituals, and what impact this primary symptom has on severity and course of illness. Mental rituals, or compulsions without overt signs, represent unique clinical challenges but often go understudied for numerous methodological and clinical reasons. Methods: In this study, we explored the impact of primary mental rituals on clinical severity and chronicity in a large, longitudinal sample of OCD patients (N 5 225) over 4 years. Results: Mental rituals were a primary presenting symptom for a sizable percentage of the sample (12.9%). Primary mental rituals were associated with greater clinical severity and lower functioning at intake, as well as a more chronic course of illness, as participants with primary mental rituals spent nearly 1 year longer in full DSM-IV criteria episodes over the 4-year follow-up interval than OCD patients without mental rituals. Conclusions: These results suggest that mental rituals are uniquely impairing and highlight the need for further empirical exploration and consideration in treatment. Depression and Anxiety 28:892-898, 2011.