The association between conduct disorder (CD) symptoms and marijuana use (MU) was assessed in 1,480 adolescent twins participating in the National Longitudinal Study on Adolescent Health. A strong association was found between CD and MU for both males (r = .38, P < .01) and females (r = .31, P < .01). Structural equation modeling on age-corrected rank normalized scores using the program Mx indicated significant gender differences in the genetic and environmental contributions to MU. Additive genetic influences were comparable in size for males (29%) and females (24%). However, for females the influence of common environmental influences was greater (58%) than for males (36%). There was also evidence of greater common environmental influences for females, as well as greater genetic influences for males for CD, but these differences did not reach significance. In addition, for CD common environmental influences did not differ significantly from zero. Additive genetic influences accounted for 61% of the variance in CD, with the remainder being of unique environmental origin. Bivariate genetic analyses suggested a moderate genetic correlation between CD and MU (rg = .28) and a low unique environmental correlation (re = .14). These results suggest that CD and MU share genetic influences, while environmental influences tend to be more specific to each behavior. These findings may have implications for the prevention and treatment of CD and substance use among adolescents. © 2002 Wiley-Liss, Inc.