Obsessive-compulsive disorder (OCD) is a polygenic neuropsychiatric disorder characterized by repetitive thoughts and behaviors that cause distress. The pathogenic repeat expansion [GGGGCC](n). found at the C9orf72 locus is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), and has also been documented in patients with psychosis and schizophrenia. Furthermore, obsessions and compulsions have been identified in patients diagnosed with ALS and/or FTD and carrying the pathogenic repeat expansion. Here, we performed genetic screening for the C9orf72 repeat expansion on 573 patients diagnosed with OCD. None of the patients were found to carry the expansion. The results show that patients with OCD do not commonly carry the pathogenic repeat expansion and therefore should not be routinely screened. OCD and psychotic patients who do test positive for the C9orf72, however, should be closely observed for the later development of FTD and ALS. Published by Elsevier B.V.