Adolescents are prone to risk-taking behaviors leading to adverse consequences such as substance abuse, accidents, violence, and victimization. However, little is known about the contribution of genetic and environmental factors to individual differences in the propensity for risk-taking. This study investigated developmental changes, longitudinal stability, and heritability of risk-taking using data from 752 adolescent twins including 169 MZ and 203 DZ pairs. The Balloon Analogue Risk Task (BART), an experimental behavioral measure of risk taking, was administered to the twins at age 12 and then re-administered to a part of this sample at age 14. Risk-taking increased with age, but individual differences showed a significant longitudinal stability. Genetic model fitting showed that at age 12, heritability of risk-taking was modest but significant in both sexes, whereas at age 14, heritability increased to 55% in males and became nonsignificant in females. The findings suggest that propensity for risk-taking as measured by BART can be a useful endophenotype for genetic studies of adolescent externalizing psychopathology, however, the utility of this measure may be limited by sex differences in heritability.