Objective: Experimental studies have shown that 2 emotion regulation strategies—suppression and reappraisal—are associated with differential profiles of physiological activation in response to a stress test. The present study aims to add to those findings by investigating whether individual differences in trait emotion regulation strategies are associated with diurnal cortisol patterns in a naturalistic context. Method: A sample of 46 men and women from the Midlife in the United States II (MIDUS II) study completed the Emotion Regulation Questionnaire (ERQ) and provided 4 salivary cortisol samples per day over 4 consecutive days. Trait reappraisal and suppression were tested as predictors of 3 cortisol parameters averaged across days: cortisol awakening response (CAR), diurnal cortisol slope (DCS), and area under the curve with respect to ground (AUCg). Results: Higher scores on the suppression scale were associated with more physiological activation, as indicated by steeper CAR and flatter DCS. Suppression was not associated with AUCg, and reappraisal was not predictive of any cortisol parameter. Conclusions: Individual differences in suppression, but not reappraisal, were linked to greater cortisol activation in this naturalistic study. These preliminary results add to a growing body of findings that link suppression to adverse psychological and physiological profiles.