Diabetes mellitus affects not only life expectancy but also quality of life. The Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial's health-related quality of life (HRQOL) and cost-effectiveness components will enable the assessment of the relative importance of the various outcomes from the point of view of patients, provide an understanding of the balance between the burdens and benefits of the intervention strategies, and offer valuable insights into adherence. The HRQOL measures used include the Diabetes Symptoms Distress Checklist; the 36-Item Short Form Health Survey, Version 2 (SF-36) (RAND Corporation, Santa Monica, CA); the Patient Health Questionnaire (PHQ) depression measure (Pfizer Inc, New York, NY); the World Health Organization (WHO) Diabetes Treatment Satisfaction Questionnaire (DTSQ); and the EuroQol Feeling Thermometer (EuroQol Group, Rotterdam, Netherlands). The cost-effectiveness analysis (CEA) in ACCORD will provide information about the relative economic efficiency of the different interventions being compared in the trial. Effectiveness will be measured in terms of cardiovascular event-free years gained and quality-adjusted life-years gained (using the Health Utilities Index Mark 3 [HUI-3] [Health Utilities Inc., Dundas, Ontario, Canada] to measure health-state utility). Costs will be direct medical costs assessed from the perspective of a single-payer health system collected by means of patient and clinic cost forms and hospital discharge summaries. The primary HRQOL and CEA hypotheses mirror those in the main ACCORD trial, addressing the effects of the 3 main ACCORD interventions considered separately. There are also secondary (pairwise reference case) comparisons that do not assume independence of treatment effects on HRQOL. CEA will be done on a subsample of 4,311 ACCORD participants and HRQOL on a subsample of 2,053 nested within the CEA subsample. Most assessments will occur through questionnaires at baseline and at 12, 36, and 48 months.