The objective of this project is to provide a better understanding of the intricate and complex reciprocal relationship between diabetes and depression, in order to provide more effective clinical care to the 1,600,000 million Americans who suffer from both these chronic conditions. This four-year project has three specific aims (1) To understand how diabetes mellitus in adults affects the onset and management of clinically significant depression, (2) To understand how clinically significant depression affects the management, progression, and outcomes of diabetes mellitus in adults, and (3) To compare the relative strength of influence of depression on diabetes and diabetes on depression. The project is conducted at Health Partners Medical Group in Minnesota, and takes advantage of sophisticated chronic disease surveillance systems already in place in a well-defined primary care population of 175,000 adults. An incident cohort of 1,000 adults with diabetes and a prevalence cohort of 3,000 patients with diabetes are each age-, gender-, and clinic/appointment date matched to a study subject without diabetes. These cohorts are tracked over a 4-year period to enable specification of temporal relationships. Linear regression, logistic regression, and structural equation modeling are used to test seven specific hypotheses, which will advance our understanding of how to improve the care provided to patients with both conditions. Strengths of the study include the following: (a) an experienced research team with a proven research record in diabetes, depression, quality improvement, and health services research; (b) a well-characterized study population; (c) prior development of validated methods to identify diabetes and depression, and measures of comorbidity, pharmaceutical treatment, medication adherence, and utilization of care using automated data, and (d) prior success improving diabetes quality of care using a similar set of strategies. This project will provide important missing information needed to improve the care of millions of people worldwide who suffer the devastating consequences of comorbid diabetes and depression. [unreadable] [unreadable]