This study examined psychosocial influences on hypothalamic-pituitary-adrenal axis activity in 105 4-year-old children transitioning to primary school. Measuring before, during, and after school transition over a period of up to 12 months, salivary cortisol was assessed on awakening and early evening. Parents reported child temperament and teachers recorded adaptive behovior Whilst cortisol at awakening and early evening increased from baseline to school transition, effects were not significant. A significant decrease occurred between transition and follow-up and front baseline to follow-up for both awakening and evening cortisol. Poorer effortful control was associated with high morning and steeper diurnal slope of cortisol at transition whilst surgency/extroversion was associated individually with greater morning and evening cortisol at transition and adaptation. Greater increase in internalizing social isolation during the first 6 months of school in more surgent/extrovert children predicted higher morning and evening cortisol at follow-up. This study is the first to explore these adaptive relationships over a 12-month period and supports social isolation over time as a key element in developmental endocrine activation.