The prevalence of renal dysfunction in patients hospitalized with acute decompensated heart failure remains poorly characterized. Data from 118,465 hospitalization episodes were evaluated. Glomerular filtration rate (GFR) was estimated using the abbreviated Modification of Diet in Renal Disease formula. At admission, 10,660 patients (9.0%) had normal renal function (GFR > or = 90 mL x min x 1.73 m2), 32,423 patients (27.4%) had mild renal dysfunction (GFR 60-89 mL x min x 1.73 m2), 51,553 patients (43.5%) had moderate renal dysfunction (GFR 30-59 mL.min.1.73 m2), 15,553 patients (13.1%) had severe renal dysfunction (GFR 15-29 mL x min x 1.73 m2), and 8276 patients (7.0%) had kidney failure (GFR < 15 mL x min x 1.73 m2 or chronic dialysis). Despite this, only 33.4% of men and 27.3% of women were diagnosed with renal insufficiency. Diuretic dose, inotrope use, and nesiritide use increased, whereas angiotensin-converting enzyme inhibitor or angiotensin II receptor blocker use decreased, with increasing renal dysfunction (all P < .0001 across stages). In-hospital mortality increased from 1.9% for patients with normal renal function to 7.6% and 6.5% for patients with severe dysfunction and kidney failure, respectively (P < .0001). The majority of patients admitted with acute decompensated heart failure have significant renal impairment, which influences treatment and outcomes.