["Sociology of Health & Illness, EarlyView. ", "\nAbstract\nDespite numerous studies on social relationships and health, the empirical focus has often been on middle‐aged or older adults, even though young adulthood is a period of considerable change in social networks. We investigated whether the associations between social relationships and allostatic load, a multisystem physiological dysregulation index that reflects chronic stress responses, vary by type of relationship and stages of the lifecourse. Relationships with spouse/partner, immediate family and friends were assessed in terms of emotional support and social strain. Poisson regression models on multiple imputed data sets from waves 2–3 (2010–2012) of the UK Household Longitudinal Study (N = 10,380) were estimated. Social strain, particularly from partners and immediate family, appeared to elicit greater stress related dysregulation during early adulthood (age 21–34 years), corresponding to a predicted difference in the allostatic load index (range 0–12) between high and low strained relationships of 1.1 (95% CI: 0.5–1.6) among young women and 0.6 (95% CI: −0.04 to 1.2) among young men. There was little evidence of an association between allostatic load and any of the social relationships among older men and women. Models of social relationships over the lifecourse need to take account of how stressful social relationships become biologically embedded in early adulthood.\n"]