We investigated associations between retrospectively assessed timing of pubertal development, interpersonal interactions, and hypothalamic-pituitary-adrenal axis reactivity to an interpersonal stress task in 110 young adult women. Participants provided salivary cortisol samples at points prior and subsequent to a video-taped conflict discussion with their romantic partner. Participants also provided subjective global ratings of their discussion on dimensions of conflict and support. For earlier developing girls, higher levels of interpersonal conflict were associated with greater physiological stress in anticipation of the discussion task and less physiological recovery following the discussion. In contrast, for later developing girls, low levels of conflict were associated with greater anticipatory stress and less physiological recovery. These findings have implications for understanding the influence of off-time pubertal development on the lifetime development of young women.