This longitudinal study examined the interrelationships between early and/or middle adolescent attention deficit hyperactivity disorder (ADHD), middle adolescent conduct disorder (CD), and later adult smoking behavior. This is a prospective longitudinal study. Data were collected via structured interviews of representative families in the northeastern United States (N = 641). The mean ages of the offspring were as follows: 14 years (T2, 1983), 17 years (T3, 1985-1986), and 32 years (T6, 2002). The dependent variable was the participants' daily cigarette smoking in their early thirties. Logistic regression analyses indicated that the relationship between ADHD and daily smoking behavior was mediated by CD with control on gender, age, SES, and adolescent smoking. CD had a direct effect on daily smoking in adulthood. Our findings suggest that ADHD is related to CD, which in turn is associated with daily smoking. Therefore, interventions with ADHD adolescents who have ADHD at an early age might lead to some reduction in later smoking provided that the intervention has a positive effect on CD. For those adolescents who never had ADHD, our findings suggest that prevention or treatment aimed at reducing CD may be most successful in reducing daily smoking later in adulthood.