Despite rapid advances in the clinical and psycho-educational management of diabetes, the quality of care received by the average patient with diabetes remains lackluster. The "collaborative" approach--the Breakthrough Series (BTS; Institute for Healthcare Improvement [IHI]; Boston)--coupled with a Chronic Care Model was used in an effort to improve clinical care of diabetes in 26 health care organizations. Descriptive and pre-post data are presented from 23 health care organizations participating in the 13-month (August 1998-September 1999) BTS to improve diabetes care. The BTS combined the system changes suggested by the chronic care model, rapid cycle improvement, and evidence-based clinical content to assist teams with change efforts. The characteristics of organizations participating in the diabetes BTS, the collaborative process and content, and results of system-level changes are described. Twenty-three of 26 teams completed participation. Both chart review and self-report data on care processes and clinical outcomes suggested improvement based on changes teams made in the collaborative. Many of the organizations evidencing the largest improvements were community health centers, which had the fewest resources and the most challenged populations. The initial Chronic Illness BTS was sufficiently encouraging that replication and evaluation of the BTS collaborative model is being conducted in more than 50 health care systems for diabetes, congestive heart failure, depression, and asthma. This model represents a feasible method of improving the quality of care across different health care organizations and across multiple chronic illnesses.