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Recent biosocial theories postulate that both biological risk and the social context influence the development of mental health problems [Boyce and Ellis (2005) Development and Psychopathology, 17(2), 271–301]. Guided by this framework, we examined whether basal cortisol and its diurnal rhythm were associated with mental health symptoms in early adolescence. Because cross‐sectional and longitudinal investigations sometimes reveal different cortisol–mental health associations, we examined the association both concurrently and longitudinally when children transition to middle school, a time which entails a major change in social context from single to multiple teachers, classrooms, and sets of classmates. Salivary cortisol was measured three times a day (waking, afternoon, and bedtime) across 3 days when adolescents were 5th graders. Mental health was measured when adolescents were in 5th and 7th …