This research investigated longitudinal associations between social activity during college (a phase of early adulthood singled out among developmental theorists as crucial to relationship development), and social, emotional, and health outcomes in midlife. Men and women age 48–50 ( n = 133) were recruited from a pool of University of Rochester graduates who completed two-weeks of event-contingent social interactions records during college in the late 1970s. College interaction diaries provided measures of structural social integration (i.e., interaction quantity), and functional perceived support (i.e., interaction quality), which were simultaneously tested as predictors of midlife health and well-being. Recruited participants reported on current social relationship quality, emotional adjustment, and physical health, and provided a series of saliva samples to capture diurnal cortisol rhythm. Midlife social relationship quality, emotional adjustment, and self-reported health were positively interrelated. Longitudinally, college social interaction quantity predicted enhanced social (friendship quality, intimacy with romantic partner, social network size), and emotional (diminished negative affectivity, enhanced positive affectivity and self- actualization) outcomes 30 years later, in midlife. Perceived social relationship quality in college was longitudinally associated with enhanced social (friendship quality, romantic intimacy), and health (better cardiovascular health) outcomes in midlife. The discussion focuses on possible mechanisms responsible for longitudinal associations that span 30 years, the relative impact of structural versus functional support, and the general invariance of these effects across sex.