Background. This and the companion paper present two sequential family studies of obsessive–compulsive disorder (OCD) conducted by the same research group, but with different sampling and best-estimate procedures. In addition to providing further data on familial transmission of OCD, we used comparison of disparate findings (moderate, specific familial aggregation in this first study versus a stronger effect for other anxiety disorders than for OCD alone in the second) to examine possible effects of proband characteristics and informant data on outcome.

Method. In this initial study we interviewed 179 first-degree relatives of 72 OCD probands and 112 relatives of 32 never mentally ill (NMI) controls. Informant data were obtained on an additional 126 relatives (total ‘combined’ samples of 263 and 154 respectively). Analyses used best-estimate diagnoses made by consensus of two ‘blinded’ senior clinicians who reviewed all diagnostic materials including proband informant data about relatives.

Results. Significantly higher risk for OCD but not other anxiety disorders was found in relatives of OCD probands compared to relatives of controls in both the directly interviewed and combined samples. There was no relationship between proband age at onset of OCD and strength of familial aggregation.

Conclusions. These data indicate moderate familial aggregation of OCD, but do not support increased transmission by early onset probands, or a familial relationship between OCD and other anxiety disorders with the possible exception of generalized anxiety disorder.