This study uses data (N = 3,088) from the first two waves of the National Survey of Families and Households to explore the effects of the transition to fatherhood and the addition of subsequent children on men's psychological and physical health, contacts with extended family, social interactions, and work behaviors. The results suggest that new children lead to changes in men's well-being and social participation, but the effects are conditioned by the structural context of fatherhood and men's initial fatherhood status. Overall, the evidence indicates that the transition to parenthood and the addition of subsequent children primarily transform the organization of men's lives, especially when they become coresident fathers. Men making the transition to parenthood are most likely to be affected. Fatherhood encourages men to increase intergenerational and extended family interactions, participation in service-oriented activities, and hours in paid labor--at the expense of spending time socializing. (Contains 4 tables.)