AIMS: To investigate the associations between glucose metabolism, left ventricular (LV) contractile reserve, and exercise capacity in patients with chronic systolic heart failure (HF). METHODS AND RESULTS: From an outpatient HF clinic, 161 patients with systolic HF were included (mean age 70 ± 10 years, 69% male, 59% had ischaemic heart disease, mean LV ejection fraction (LVEF) 37 ± 9%). Thirty-four (21%) patients had known diabetes mellitus (DM). Oral glucose tolerance testing (OGTT) classified patients without a prior DM diagnosis as normal glucose tolerance (NGT), impaired glucose tolerance or new DM. All patients completed low-dose dobutamine echocardiography (LDDE) and 154 patients a 6-min walking distance test (6MWD). Compared with patients with NGT, patients with known DM had lower resting LVEF (33.4 vs. 39.1%, P <0.05) and higher E/e' (13.9 vs. 11.4, P <0.05). During LDDE, an increase in LVEF could be observed in all glycemic groups (mean 8.2% absolute increase), but the contractile reserve was lower in patients with known DM (-5.4%, P = 0.001) and new DM (-3.5%, P = 0.035) compared to patients with NGT. 6MWD was lower in known DM (349 m) and new DM (379 m) compared with NGT (467 m) (P <0.001). Differences in clinical variables, resting echocardiographic parameters or contractile reserve, did not explain the exercise intolerance related to diabetes. CONCLUSION: Diabetes, known or newly detected by OGTT, is independently associated with reduced LV contractile reserve and exercise intolerance in outpatients with systolic HF. These findings may offer one explanation for the excess mortality related to diabetes in HF.