The reliability and validity of the Yale-Brown Obsessive-Compulsive Scale were examined according to a multi-trait multi-method approach in a sample of 54 outpatients with obsessive-compulsive disorder (OCD). Internal consistency was acceptable but was improved by deletion of items concerning resistance to obsessions and compulsions. Inter-rater reliability was excellent, but test-retest reliability over an average interval of 48.5 days was lower than desirable. The YBOCS demonstrated good convergent validity with most other measures of OCD, but divergent validity vis à vis depression was poor. Analyses of new items assessing avoidance and the duration of obsession-free and compulsion-free intervals indicated that only the avoidance rating added meaningfully to the full scale score. In future research the authors recommend deletion of the resistance items and inclusion of the avoidance item to yield a revised 9-item YBOCS total score.