Adrenocortical attunement-similarity in hypothalamic-pituitary-adrenal (HPA) axis activity-has been well-documented in close relationships (e.g., between romantic partners, parents and children, and close friends). However, little is known about adrenocortical attunement during early relationship formation. In the current study, we examine dyadic adrenocortical attunement during a guided conversation in which two new acquaintances (N = 140 people, 70 dyads), who were university students or adults in the community, answered questions about themselves. Dyads were randomly assigned to answer questions designed to elicit dyad members to reveal a high or low amount of personal information (i.e., to self-disclose at high or low levels). We collected saliva samples (assayed for cortisol) before and after the conversation, and we coded behavioral self-disclosure-the extent to which people revealed their thoughts, feelings, and facts about themselves-during the conversation. As expected, dyads who were assigned to ask and answer high self-disclosure questions disclosed more than those assigned to ask and answer low self-disclosure questions. In addition, greater self-disclosure during the conversation was associated with greater similarity in cortisol change-that is, dyad members who revealed more about themselves experienced more similar cortisol changes in response to their conversation. This work reveals one social process through which adrenocortical attunement occurs during early relationship formation, and, in doing so, describes how our physiological functioning is linked to those around us-even people we have just met.