This study examined the differential effects of parenting and sibling influences (and the interaction between the two) in predicting adolescents' problem behaviors. The reliability and validity of the Sibling Relationship Questionnaire was also assessed. Subjects were 99 intact families from the San Francisco Bay area, all of whom contained both parents and at least one target child between 10 and 18 years of age still living in the home. Analysis indicated that sibling behavior was a significant predictor of deviant behavior, as measured by the Child Behavior Checklist, even when parenting effects were statistically controlled. Parenting effects had a separate effect on adolescents' positive behaviors. The analysis of the Sibling Relationship Questionnaire indicated adequate test-retest and internal consistency reliability, and construct validity. Findings are discussed in terms of the larger literature on socialization on the different ways that siblings and parents influence adolescent outcomes.