BACKGROUND: Among patients with chronic medical conditions, unrelated conditions are often undertreated. OBJECTIVE: To compare the quality of diabetes care delivered to diabetic patients with and without cancer in a large regional integrated delivery system. DESIGN: Observational cohort study using propensity score methods to control for baseline differences between diabetic patients with and without a history of cancer. SUBJECTS: A total of 5773 Kaiser Northern California members with diabetes and previous cancer and 23,092 members with diabetes and no previous cancer. MEASURES: : Nine measures of diabetes technical quality and clinical outcomes in 2003. RESULTS: : Relative to diabetic patients without cancer, those with cancer had higher adjusted rates of HbA1c testing (66.3% vs. 64.4%; P = 0.02), HbA1c control (73.4% vs. 70.9%; P < 0.001), and urine microalbumin testing (59.1% vs. 55.2%; P < 0.001) but lower rates of low-density lipoprotein (LDL) cholesterol control (40.7% vs. 42.2%; P = 0.02) and statin use if LDL >100 mg/dL (76.7% vs. 80.6%; P < 0.001). The groups had similar rates of LDL cholesterol testing, dilated retinal examinations, blood pressure control, and angiotensin converting enzyme (ACE) inhibitor use for hypertension (all P >/= 0.20). CONCLUSIONS: Despite the potential for cancer-related services to compete with delivery of diabetes care, diabetic patients with cancer received care of generally similar quality relative to diabetic patients without cancer in this integrated delivery system. Nevertheless, the quality of diabetes care delivered to all patients could be improved, particularly the control of LDL cholesterol and blood pressure. Combining data from electronic disease registries has the potential for monitoring quality of care delivered to patients with more than 1 major medical illness.