OBJECTIVES: The objective was to measure the difference between optimal patient care and actual patient care in family practices before and after the introduction of a computer decision support chronic disease management system (CDMS) for diabetes. METHODS: This 1-year, prospective, observational, pre/post study evaluated the use of a CDMS with a diabetes patient registry and tracker in family practices in patient enrolment models. Aggregate group practice-level data from all rostered diabetes patients were analyzed. The primary outcome was the change in the proportion of patients with up-to-date “ABC” monitoring frequency (i.e., HbA1c, blood pressure, and cholesterol). Changes in practice care and treatment elements (e.g., retinopathy screening) were also determined. Usability and satisfaction with the CDMS were measured. RESULTS: Nine sites, 38 healthcare providers and 2,320 diabetes patients were included. The proportion of patients with up-to-date HbA1c (45%), cholesterol (38%), and ABC (12%) monitoring did not change. The proportion of patients with up-to-date blood pressure monitoring improved, from 16% to 20%. Data on foot examination, retinopathy screening, and use of angiotensin-converting enzyme inhibitors or angiotensin-receptor blockers were not available or not up-to-date at baseline for 98% of patients. By the end of the study, health care providers tended to be more negative towards the Usefulness, Training, Daily Practice, and Support domains of the CDMS, but there was a more positive response for the System, Using, Practice Planning, Satisfaction, and Learning domains. CONCLUSIONS: This real-world evaluation of a web-based CDMS for diabetes failed to impact physician practice due to limited use of the system.