•We model disagreeableness through observations with friends in adolescence.•These observations are to predict future behavior in friendships and romantic relationships.•Disagreeableness predicts future partner reports of poorer relationship quality.•Disagreeableness does not predict individual’s own reports of relationships.•We propose this relationship blindness to explain disagreeableness stability. The ability to form and maintain relationships with friends and romantic partners is a major developmental task for adolescents. Disagreeable youth are likely to struggle with this task, yet little is known about how they maintain their oppositional style from adolescence to adulthood. The current study examines the long-term implications of disagreeableness in a diverse sample of 164 adolescents assessed repeatedly across a 10-year period along with their friends and romantic partners. Disagreeableness at age 14 and 15 was assessed in observation with friends. Disagreeableness was then examined as a predictor of both future relationship quality with friends at age 16 and romantic relationships at age 21. The results indicate that although disagreeable youth do not report any relationship struggles, both their friends and romantic partners see their relationships as being low in quality. Findings suggest a developmental process by which disagreeable adolescents maintain their oppositional style through a mechanism of relationship blindness, as they simply are unable to see the relationship issues that their friends and partners clearly perceive.