Cystatin C is a novel marker of renal function that has been found to predict adverse cardiovascular outcomes in ambulatory patients. The aim of this study was to investigate whether this biomarker predicts the length of hospitalization and adverse outcomes in patients hospitalized for heart failure. Two hundred forty consecutive patients aged ≥25 admitted to Johns Hopkins Hospital with exacerbations of heart failure were prospectively enrolled. Cystatin C levels were measured on admission. Patients were followed for 1 year. The primary outcome measure was the length of hospitalization. Secondary outcomes included all-cause mortality and readmission for heart failure. Cystatin C showed no significant association with the length of hospitalization. Patients in the highest quartile (quartile 4) of cystatin C level were at increased risk for death (hazard ratio 2.07 for quartile 4 vs quartiles 1 to 3, p = 0.01) and death or rehospitalization (hazard ratio 1.61 for quartile 4 vs quartiles 1 to 3, p = 0.01). The association between cystatin C and the combined end point of death or rehospitalization during 1-year follow-up remained significant after adjusting for age, race, gender, co-morbidities, and creatinine. Cystatin C was more predictive of these end points than creatinine, and the combination of cystatin C and creatinine was more predictive than either variable alone. In conclusion, cystatin C may be useful in addition to creatinine for predicting outcomes after admission for acute heart failure exacerbations.