OBJECTIVE - To investigate whether monitoring and discussing psychological well-being in outpatients with diabetes improves mood, glycemic control, and the patient's evaluation of the quality of diabetes care. RESEARCH DESIGN AND METHODS - This study was a randomized controlled trial of 461 outpatients with diabetes who were randomly assigned to standard care or to the monitoring condition. In the latter group, the diabetes nurse specialist assessed and discussed psychological well-being with the patient (with an interval of 6 months) in addition to standard care. The computerized Well-being Questionnaire was used for this purpose. Primary outcomes were mood, HbA1c, and the patient's evaluation of the quality of diabetes care at 1-year follow-up. The number of referrals to the psychologist was analyzed as a secondary outcome. Intention-to-treat analysis was used. RESULTS - The monitoring group reported better mood compared with the standard care group, as indicated by significantly lower negative well-being and significantly higher levels of energy, higher general well-being, better mental health, and a more positive evaluation of the quality of the emotional support received from the diabetes nurse. The two groups did not differ for HbA1c or in their overall evaluation of the quality of diabetes care. In the monitoring condition, significantly more subjects were referred to the psychologist. CONCLUSIONS - Monitoring and discussing psychological well-being as part of routine diabetes outpatient care had favorable effects on the mood of patients but did not affect their HbA1c. Our results support the recommendation to monitor psychological well-being in patients with diabetes.