Diabetes is a pervasive metabolic disease that disproportionately affects persons with serious mental illness. The authors studied the effect of diabetes on quality of life in a sample of 369 adult outpatients with schizophrenia or major mood disorder, 201 of whom had type 2 diabetes. Patients with diabetes reported greater impairment in both physical and mental-health quality of life than those without diabetes. The diabetes patients also reported less satisfaction with health but not with other life domains. Medical providers need to be attentive to the burden of disease experienced by patients with both serious mental illness and diabetes.