Contracting a sexually transmitted disease (STD) is one of the most serious public health issues for adolescents and young adults; rates of STDs among American Indian youth are among the highest of any racial or ethnic group in the United States. Although it is one of the key risk factors for spreading STDs, little is known about individual development of multiple sexual partners in any ethnic group. Drawing from social cognitive theory, this study used latent growth curve modeling with a sample of 518 American Indian adolescents and young adults to delineate the relationship across 4 years between youths' confidence in resisting risk and the number of sexual partners they had. The development of resistive efficacy was similar across genders: Although women reported higher levels at all time points, both genders increased across time. Men reported relatively stable levels of sexual partners; women had fewer sexual partners initially, but the number increased significantly and then decelerated. Further, the 2 trajectories had significant interdependence: Both intercepts and slopes were negatively related to each other across construct, suggesting that increasing resistive efficacy was related to decreasing number of sexual partners. (Contains 4 tables and 2 figures.)