Abstract Objectives. Diabetes mellitus (DM) in combination with heart failure (HF) is associated with a high risk of death, but it is uncertain whether the prognosis differs in ischemic and non-ischemic HF groups. Design. One thousand, three hundred and six patients with ischemic HF and 1315 patients with non-ischemic HF were followed for 6.8 years. Risk of all caue mortality was analyzed using Cox proportional hazard models. Results. Mean age was 73 (+/- standard deviation 11) years. Two hundred and forty eight of 1306 (19%) patients with ischemic HF and 172/1315 (13%) patients with non-ischemic HF had DM (p for difference < 0.0001). Overall, 939/1306 (72%) patients in the ischemic HF group, and 835/1315 (64%) patients in the non-ischemic HF group died, respectively. Totally 191/248 (77%) DM patients with ischemic HF, and 127/172 (74%) DM patients with non-ischemic HF died. DM was associated with adjusted hazard ratios (HR) of 1.45 (1.22-1.73, p < 0.0001) if ischemic HF, and 1.50 (1.22-1.84, p < 0.0001) if non-ischemic HF (p for interaction = 0.4), compared to non-DM non-ischemic HF patients. Conclusion. The long-term prognosis of DM is equally adverse in ischemic and non-ischemic HF patients.