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Obsessive compulsive disorder (OCD) is a complex psychiatric disease characterized by recurring obsessions or compulsions that cause significant distress to the patient. The etiology of this disorder remains largely unknown, although a genetic component has been suggested. Many candidates genes have been evaluated based on a possible serotoninergic and dopaminergic brain dysfunction. We postulate the μ opioid receptor (MOR) gene as a candidate because some observations support a role of the opioid system in OCD. The opioid antagonist, naloxone, rapidly exacerbates OCD symptoms and the opioid agonist, tramadol, was reported to be effective in the treatment of some patients. We studied two single nucleotide polymorphisms (C17T and A118G) in 51 trios with OCD. Genotyping was analyzed with transmission desequilibrium test (TDT). The allelic variant +17T of the C17T polymorphism had a low frequency (1%) in our population that did not allow for statistic analysis. However, for the allelic variant +G of the A118G polymorphism we were able to performed statistical comparisons. Our results showed a trend toward significance (χ² McNemar = 3.6, P = 0.065) for TDT in patients with comorbid tics. It is an interesting finding that should be tested in a larger sample of OCD patients with tics. © 2004 Wiley-Liss, Inc.