The goal of this cross-sectional study was to identify the rates and correlates of treatment refusal and/or dropout in a treatment-seeking sample of patients with obsessive-compulsive disorder (OCD). Specifically, we investigated the relationships between treatment adherence and different OCD dimensions, intelligence, and insight into OCD. The study involved 60 patients with OCD who were being treated in a specialized university OCD clinic. The patients' adherence to standard treatment was assessed with the Treatment Adherence Survey-Patient Version. Patients were also evaluated with the following instruments: the Mini-International Neuropsychiatric Interview 6.0, the Dimensional Yale-Brown ObsessiveCompulsive Scale-short version, the Brown Assessment of Beliefs Scale, the Beck Depression Inventory, the Sheehan Disability Scale, and the Wechsler Abbreviated Scale of Intelligence. The patients with OCD who refused to undertake CBT (46%) displayed greater rates of obsessions with aggressive/violent content. Among patients who started CBT (n=32), 51% withdrew before completing therapy. Patients who refused medication for OCD (52%) displayed greater severity of OCD (particularly hoarding), less insight into symptoms, and greater disability. Of the patients with OCD who were given drug therapy (n=58), 61% reported having taken their medication less frequently and/or at a smaller dose than prescribed or discontinuing the use of medication altogether. Treatment nonadherence is common among patients with OCD. This study found that aggressive/violent obsessions were associated with nonadherence to CBT, while greater severity of OCD (particularly hoarding) and poorer insight were associated with poorer adherence to drug therapy. Future research is needed to clarify whether these OCD phenotypes predict or are the consequence of treatment nonadherence.