The authors assessed the relationship between glycemia and length of hospital stay (LOS) in a prospective cohort study of patients with diabetes mellitus and heart failure (HF). Of 212 patients with acute HF exacerbation, 119 (56%) also had diabetes. The mean age of the cohort was 63+/-0.87 years, and the mean body mass index was 29.3 kg/m2. Diabetic patients had significantly longer LOS compared with the nondiabetics (5.0+/-0.29 vs 3.4+/-0.19; P<.001). In patients with diabetes, the mean glycated hemoglobin A1c was 8.3%, admission blood glucose (BG) was 169+/-7.7 mg/dL, and average BG was 196+/-8.1 mg/dL. After adjusting for age, sex, weight, hypertension, renal function, and anemia, LOS was significantly correlated with admission BG (r=0.31; P<.001) and average BG (r=0.34; P=.001). In patients with acute HF exacerbation, diabetes significantly prolonged LOS. Hyperglycemia correlated with LOS.