The presence of diabetes mellitus (DM) adversely affects the natural history of heart failure (HF), but its prognostic significance is unknown in acute decompensated HF. Of the 498 patients enrolled with decompensated HF requiring intravenous vasoactive therapy, 236 (47.4%) had a previous diagnosis of DM. After 6 months, 113 patients (22.7%) had died. A Cox proportional-hazards model showed a significant association between DM and worse survival after hospital discharge. DM is common among patients admitted with decompensated HF, and diabetes-related biologic differences contribute to the progression of HF.