This study prospectively examines the association between discord with in-laws and the long-term relationship success of husbands and wives who had been married for an average of almost 2 decades. We hypothesized that the quality of spouses' relationships with their parents-in-law would predict spouses' marital success. In addition, this study underscores the causal role of in-laws by examining the influence of marital success on relationships with in-laws. For wives, discord with mothers- and fathers-in-law predicted own perceptions of marital success at a later time period. Results were slightly different for husbands' discord with fathers-in-law. The reverse (marital success predicting less discord with in-laws) was only true for husbands. The study also explored the influence of spouses' discord with in-laws on partners' perceptions of marital success. These are among the first prospective, longitudinal findings demonstrating that, even in long-term marriages, conflicts in extended family relations will erode marital stability, satisfaction, and commitment over time.