Abstract: Aims To test the hypothesis that diabetic status may be used as a prognostic indicator in heart failure (HF) patients. Methods and results We studied 1246 consecutive patients with left ventricular dysfunction. All patients had a cardiopulmonary exercise test and an echocardiogram. Cardiac catheterisation was systematically performed to define HF aetiology. Twenty-two percent of the patients were diabetic (hypoglycaemic drugs or fasting blood glucose [Formula: see text] 126 mg/dL); in diabetic patients, HF aetiology was ischaemic in 58% vs. 40% in non-diabetic patients ( [Formula: see text] ). Clinical follow-up (median 1200 days) was obtained for 1241 patients. There was a statistically significant effect of diabetes mellitus on cardiac survival that differed according to HF aetiology (interaction [Formula: see text] ). Diabetes mellitus was an independent predictor of cardiovascular mortality in ischaemic patients (HR=1.54 [1.13; 2.09]; [Formula: see text] ) but not in non-ischaemic patients (HR=0.65 [0.39; 1.07]; [Formula: see text] ). When diabetic patients were defined as patients receiving hypoglycaemic drugs at baseline, diabetes mellitus remained an independent predictor of cardiovascular mortality in ischaemic patients (HR=1.43 [1.03; 1.98]; [Formula: see text] ) while diabetes mellitus was associated with a statistically significant decrease in cardiovascular mortality in non-ischaemic patients (HR=0.46 [0.23; 0.88]; [Formula: see text] ). Conclusion The prognostic impact of diabetes mellitus in HF patients is markedly influenced by the underlying aetiology and is particularly deleterious in those with ischaemic cardiomyopathy. 

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