Obsessive
-compulsive disorder (OCD) is a complex psychiatric disorder characterized by recurring obsessions or compulsions that cause significant distress to the patient or significantly interfere with the patient's normal home, work, or social activities [Diagnostic and Statistical Manual of Mental Disorders, 4th Edition. Washington, DC: American Psychiatric Association, 1994]. Twin and family studies have suggested that OCD has a significant genetic component. We performed complex segregation analyses using POINTER with families ascertained through an OCD-affected proband. In an attempt to resolve the phenotypic heterogeneity observed among individuals with OCD these segregation analyses used four factor-analytic symptom dimensions to subset the family sample based upon probands' symptom factor scores. Analysis of the entire sample allowed rejection of only the no transmission model; that model was also rejected in all subsequent analyses. Limiting the analyses to families with at least one OCD-affected member in addition to the proband (the demonstrably familial form of OCD) allowed rejection of all models except the mixed model. Analyses limited to families of high-factor-3 (symmetry and ordering symptoms) probands led to rejection of the polygenic model, indicating the involvement of a major locus. Additionally, the relative risk of OCD or subclinical OCD was 1.7 for relatives of probands with a fac-tor 3 score greater than zero compared with relatives of probands with a low factor score. The symptoms attributed to high factor 3 scores (symmetry and ordering) may constitute a genetically significant symptomatic subtype of OCD.