Objectives: Diabetes-related medical errors in outpatient practice are common and costly. This study attempts to accurately identify, classify, and interpret patterns of diabetes-related medical errors in primary care settings using diagnostic, laboratory, and pharmacy data. Methods: Automated diagnostic, laboratory, and pharmacy data were used to evaluate outpatient care received from 5,729 adults with an established diagnosis of diabetes, who received care at a single large medical group over a 12-month period of time. A subset 4,152 adults with diabetes who were (a) younger than 80 years; (b) had a Charlson comorbidity score of 2 or lower; (c) had pharmacy coverage; and (d)were linked to a primary care physician, were classified as having a glycemic-control error, a lipid-control error, or a pharmacy error. Results: Among the subset of 4,152 patients, 62.3 percent had one or more medical errors during the 1-year study period. Errors related to glycemic-control care occurred in 22.1 percent; errors related to low-density lipoprotein-cholesterol (LDL) care occurred in 58.3 percent. Inappropriate or risky drug prescribing occurred in 9.9 percent of the 5,729 patients. Metformin was used by 27.2 percent of those with congestive heart failure, and by 16.6 percent of those with a measured serum creatinine value greater than 1.5 mg/dl. Conclusions: Medical errors in adults with diabetes are the norm rather than the exception. Passive surveillance for medical errors can be done accurately and inexpensively using automated data routinely available in many primary care settings. These observed patterns of medical error define specific clinical domains and patient subgroups for whom aggressive efforts to reduce medical errors are most urgently needed.