This article focuses on the possibility that autism spectrum disorder (ASD: Asperger syndrome, autism and atypical autism) in its milder forms may be clinically important among a substantial proportion of patients with obsessive-compulsive disorder (OCD), and discusses OCD subtypes based on this proposition. The hypothesis derives from extensive clinical experience of OCD and ASD, and literature searches on MEDLINE. Neuropsychological deficits are more common in OCD than in panic disorder and depression. Moreover, obsessive-compulsive and schizotypal personality disorders are over-represented in OCD. These may constitute misperceived clinical manifestations of ASD. Furthermore, repetitive behaviours and hoarding are common in Asperger syndrome. It is suggested that the comorbidity results in a more severe and treatment resistant form of OCD. OCD with comorbid ASD should be recognized as a valid OCD subtype, analogous to OCD with comorbid tics. An odd personality, with paranoid, schizotypal, avoidant or obsessive-compulsive traits, may indicate these autistic dimensions in OCD patients. (Contains 1 table.)