A considerable body of research has extolled the virtues of establishing rapport in negotiations. Negotiators who are high in rapport tend to be more likely to reach an agreement and more satisfied with the outcome. Although rapport generally has been found to have positive effects in standard negotiation settings, we investigate the effects of rapport in impasse settings, where conflict between negotiators' core needs means that a successful deal can only be reached when one or both parties acts unethically or “misbehaves,” for example, by lying to the negotiation partner. In a series of three experiments, we find that negotiators who have a high level of rapport are more likely to behave unethically than are negotiators who have a low level of rapport. We find this effect holds both when high rapport results from the way in which negotiations are conducted (face-to-face versus computer mediated) and also when rapport is established through a brief rapport-building exercise before negotiations begin. Finally, we find that the negative effects (unethical behavior)—but not the positive effects (satisfaction with the negotiation, trust, and willingness to work in the future with the negotiation partner)—of high rapport are reduced when negotiators are given a simple reminder before negotiations begin that one's actions can have long-term repercussions for one's reputation. Taken together, this research supports the idea that, despite its several advantages, in certain situations rapport has a dark side, of which negotiators must be wary.