AIMS To study the specific impact of diabetes on long-term mortality in very old subjects with multiple comorbidities and functional disabilities. METHODS The prevalence of vascular disorders, global comorbidity load (cumulative illness rating scale [CIRS]) and functional disabilities (activities of daily living [ADL] and Lawton's instrumental ADL [IADL] scores) were determined according to diabetes status in a cohort of 444 patients (mean age 85.3±6.7 years; 74.0% women) admitted to our geriatric service. Also, the specific impact of diabetes on 4-year mortality was analyzed using Cox proportional-hazards models. RESULTS Diabetic patients had higher BMI scores (27.1±4.9 vs. 23.4±4.7 kg/m(2) in controls; P<0.001), and higher prevalences of hypertension (81.9% vs. 65.1%, respectively; P=0.003) and ischaemic heart disease (33.7% vs. 22.2%, respectively; P=0.033), but not of stroke and renal insufficiency. They also had more comorbidities (CIRS score excluding diabetes: 15.1±4.5 vs. 13.8±4.8, respectively; P=0.016) and functional disabilities. Diabetes was associated with mortality (HR: 1.42, 95% CI: 1.02-1.99; P=0.041) after adjusting for age, gender and BMI, and this persisted after adjusting for individual vascular comorbidities, but disappeared after adjusting for CIRS, ADL or IADL scores. CONCLUSION Diabetes was associated with 4-year mortality after adjusting for the inverse relationship between mortality and BMI. This association was better accounted for by the global comorbidity load and functional disabilities than by the individual vascular comorbidities. These findings suggest that the active management of all--rather than selected--comorbidities is the key to improving the prognosis for older diabetic patients.