The proposed project aims to measure the effects of increased self-monitoring of blood glucose on the cost, quality, and outcomes of diabetes care. This population-based, longitudinal study would analyze care for about 3,750 adult patients with diabetes mellitus who were continuously enrolled in Harvard Pilgrim Health Care for 24 months before and 24 months after coverage for self-monitoring began in December 1993. Using interrupted time-series analysis, the project would stratify the results by patients receiving insulin and those receiving oral hypoglycemic agents, and also examine low-income patients and those with poor glycemic control. Outcomes of interest include glycemic control, the principal outcome, as well as use of monitors, frequency of laboratory glucose testing, use of hypoglycemic agents and other medications, ambulatory and inpatient service use, and cost of treatment.