BACKGROUND: The majority of older adults have 2 or more chronic conditions and among patients with diabetes, 40% have at least three. OBJECTIVE: We sought to understand how the number, type, and severity of comorbidities influence diabetes patients' self-management and treatment priorities. DESIGN: Cross-sectional observation study. PATIENTS: A total of 1,901 diabetes patients who responded to the 2003 Health and Retirement Study (HRS) diabetes survey. MEASUREMENTS: We constructed multivariate models to assess the association between presence of comorbidities and each of 2 self-reported outcomes, diabetes prioritization and self-management ability, controlling for patient demographics. Comorbidity was characterized first by a count of all comorbid conditions, then by the presence of specific comorbidity subtypes (microvascular, macrovascular, and non-diabetes related), and finally by severity of 1 serious comorbidity: heart failure (HF). RESULTS: 40% of respondents had at least 1 microvascular comorbidity, 79% at least 1 macrovascular comorbidity, and 61% at least 1 non-diabetes-related comorbidity. Patients with a greater overall number of comorbidities placed lower priority on diabetes and had worse diabetes self-management ability scores. However, only macrovascular and non-diabetes-related comorbidities, but not microvascular comorbidities, were associated with lower diabetes prioritization, whereas higher numbers of microvascular, macrovascular, and non-diabetes-related conditions were all associated with lower diabetes self-management ability scores. Severe, but not mild, HF was associated with lower diabetes prioritization and self-management scores. CONCLUSIONS: The type and severity of comorbid conditions, and not just the comorbidity count, influence diabetes patients' self-management. Patients with severely symptomatic comorbidities and those with conditions they consider to be unrelated to diabetes may need additional support in making decisions about care priorities and self-management activities. © 2007 Society of General Internal Medicine.