We examined the development of children's positive and negative attitudes toward other-gender peers over 1 year, and explored the longitudinal social consequences of holding positive or negative attitudes on the beholder of these attitudes. Participants were 206 second graders (Mage = 7.18 yrs, SD = .56, 50% girls) and 206 fourth graders (M[subscript age] = 9.10 yrs, SD = 0.66, 44.2% girls) from diverse ethnic racial backgrounds (54.6% White; 17.2% Latinx, 4.4% Black, 5.3% Asian, 2.9% Native American, .7% Pacific Islander, 13.1% other) with average household income ranged from $51,000 to $75,000, and they were assessed in 2 consecutive years. Developmental change was assessed using latent change score analysis, which showed that positive other-gender attitudes increased over time (for boys) whereas negative other-gender attitudes decreased for everyone. Path analyses showed that both positive and negative other-gender attitudes predicted children's perceptions of stressful other-gender interactions and their inclusion expectancies by other-gender peers longitudinally, controlling for same-gender attitudes. We also examined the extent to which the predicted relation between attitudes and inclusion expectancies was mediated by children's perceptions of stressful experiences with other-gender peers. We found that the extent of mediation varied by the type of attitudes and by children's age. Overall, findings contributed to the understanding of the development of children's other-gender attitudes, and underscored the consequences of these attitudes for the beholder of attitudes. This work also sheds light on the discussion of intervention strategies aimed at improving children's gender-based intergroup relations.