Recently, psychiatrists have been paying attention to the presence of PDD（Pervasive Developmental Disorder）in difficult cases of mental disorder. We have found that patients with treatment-resistant OCD（Obsessive-Compulsive Disorder）are very likely to have PDD. If the presence of PDD can be recognized at an early stage based on the clinical features of patients with OCD, it would lead to effective treatment. However, there has been little research on the epidemiology of OCD patients with PDD, especially in adults. In this study, we investigated the percentage of PDD in adult OCD and the clinical characteristics of OC symptoms,comparing between OCD patients with or without diagnoses of PDD. The results showed that, in 48 patients with OCD, as many as 13（27%）were diagnosed with PDD（PDD＋）. We identified several items in the Y-BOCS（Yale-Brown Obsessive Compulsive Disorder）symptom checklist that showed significantly higher rates in PDD＋. In the ‘Obsession’checklist, the items were obsession with need for symmetry or exactness,needing to know or remember and bothered by certain sounds/noises and the fear of losing things. In the‘Compulsion’checklist,the items were checking,repetition,ordering/arranging, and hoarding/collecting compulsions. Among these items, needing to know or remember, hoarding/collecting compulsion, and the fear of losing things seemed to be closely related. The need to know or remember was considered to be the core factor. This relationship was found only in PDD＋, and so it may prove useful in clinical guides to identify PDD＋ among patients with OCD.