The aim of this study was to evaluate the clinical characteristics of OCD patients with poor insight, and the predictive value of poor insight with respect to response to treatment with serotonin reuptake inhibitors (SRIs). One hundred ten patients fulfilling DSM-IV criteria for OCD were included in the study and assessed by standardized instruments. Seventy-nine patients were treated with SRIs and followed prospectively for 3 years. During the follow-up period, the clinical status of each patients was evaluated monthly during the first year and bimonthly thereafter by means of the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) and the Hamilton Rating Scale for Depression (HDRS). Twenty-one percent of the patients did not recognize obsessive-compulsive symptoms as unreasonable or senseless. Patients with poor insight had a earlier age at onset, a greater severity of obsessive-compulsive symptoms at intake, a higher rate of schizophrenia spectrum disorders in first-degree relatives and a higher comorbidity rate of schizotypal or obsessive-compulsive personality disorders. At the end of the study, 62% percent of the patients with normal insight responded to SRIs, whereas none of the patients with poor insight was found to be responder. The study provides evidence that poor insight is associated with specific clinical characteristics and treatment failure in OCD. Further studies should aim at identifying additional treatment strategies that are effective in OCD patients with poor insight.