Context: The hypothalamic pituitary-adrenal axis is thought to play a role in type 2 diabetes (T2D). However, evidence for an association between cortisol and future glucose disturbance is sparse. Objective: The aim was to examine the association of diurnal cortisol secretion with future T2D and impaired glucose metabolism in a community-dwelling population. Design: This is a prospective cohort study of salivary cortisol measured at the 2002-2004 clinical examination of the Whitehall II study, United Kingdom. We measured cortisol (nmol/l) from six saliva samples obtained over the course of a day: at waking, 30 minutes, 2.5 hours, 8 hours, 12 hours, and bedtime. Participants who were normoglycemic in 2002-2004 (phase 7) were reexamined in 2012-2013 (phase 11). Setting: The occupational cohort was originally recruited in 1985-1988. Participants: A total of 3270 men and women with an average age of 60.85 years at phase 7 (2002-2004). Outcome Measures: Incident T2D and impaired fasting glucose in 2012-2013 were measured. Results: Raised evening cortisol at phase 7 was predictive of new-onset T2D at phase 11 (odds ratio [OR], 1.18; 95% confidence interval [CI], 1.01-1.37) with a trend for a flatter slope in participants with incident T2D (odds ratio, 1.15; 95% CI, 0.99-1.33). When expanDing this analysis to a broader category of glucose disturbance we found that a flattened diurnal cortisol slope at phase 7 was predictive of future impaired fasting glucose or T2D at phase 11 (OR, 1.12; 95% CI, 1.02-1.22), as was high bedtime cortisol (OR, 1.10; 95% CI, 1.01-1.20). Conclusions: In this nonclinical population, alterations in diurnal cortisol patterns were predictive of future glucose disturbance.