The present cross-cultural study compared self-reported assertiveness in 652 Swedish and 654 Turkish high school students by using a multidimensional measure called the Scale for Interpersonal Behavior (SIB). Four hypotheses were tested in the study. First, the hypothesis that Swedish adolescents would be more assertive than their Turkish counterparts was supported by the data. Second, the expectation that Turkish boys would be more assertive than Turkish girls, while there would be no differences between Swedish girls and boys, was not confirmed. In general, girls were found to be more skilled than boys in expressing and dealing with personal limitations. Third, as expected, more assertive adolescents in both Sweden and Turkey reported having more friends and receiving more social support than their less assertive peers. Finally, the data supported the expectation that older adolescents would be more assertive than younger ones. The results are discussed in terms of cultural and gender differences.