In this prospective study, we examined the link between positive family relationships during childhood and adolescence and health and happiness three decades later in middle adulthood. We also investigated the stability of positive family relationships into adulthood as one possible pathway underlying this long-term association. Data were from the Fullerton Longitudinal Study (FLS) an ongoing investigation in the United States initiated in 1979 when children were aged 1 year with the most recent data collected in 2017. A cross-informant methodology was employed in which mothers and children independently completed the Positive Family Relationships (PFR) scale annually when children were of ages 9-17 years. When study children reached age 38, they reported on their current PFR, global health, and comprehensive happiness. Structural equation models revealed that children's perceptions of the family during childhood and adolescence predicted both their health and happiness at age 38. Mothers' perspectives of PFR predicted greater adult children's health, but did not predict their happiness. Associations were independent of family socioeconomic status, gender, intelligence, and extraversion. Moreover, while controlling for behavior problems (proxy for health) and happiness at age 17, both children's and mothers' early PFR related to PFR at 38 years, which in turn, predicted increased health and happiness at age 38, thus providing evidence for a pathway underlying this long-term connection. Our prospective findings revealed that families in which members get along well and support each other during the childhood and adolescent years furnish a foundation for positive family relationships in adulthood, which are associated with greater health and happiness. (PsycInfo Database Record (c) 2022 APA, all rights reserved).