Using approximately 2,000 sibling pairs from the ab] National Longitudinal Survey of Labor Market Experience of Youth, we examine the influence of an older sibling’s age at first sexual intercourse on the sexual initiation of a younger sibling. Hypotheses about differences by gender composition of the pair are tested, using a framework derived from social comparison theory and a two-stage failure-time model. Results provide evidence of a direct but modest-sized older sibling effect for white but not black youth. This effect is approximately equal in magnitude for same- and oppositesex siblings. Little support is offered for the greater salience and association of sexual activity for brother-brother as compared with sister-sister pairs.