Obsessive-compulsive disorder (OCD) is a severe, heritable condition with a lifetime prevalence of about two percent of the population. The mode of inheritance is poorly understood but is likely complex, involving multiple loci of small to major effect. Since 1995, the NIMH-IRP has been active in a multi-center family study of OCD, led by Dr. Gerald Nestadt of Johns Hopkins University, which was approved via a competitive NIMH extramural application (MH 502140). An expanded consortium of sites (including new sites at Brown and Harvard Universities) anticipates adding 300 new affected sib-pair families over the next three years. This sample will be used for linkage and association analyses. Data will be shared within this consortium of investigators studying OCD, and will eventually be combined with data obtained from a second consortium.