Despite the potential importance of a rising creatinine level in patients hospitalized for heart failure, there is little information about factors that may predispose patients to this condition or its association with outcomes. We sought to determine the incidence and identify factors associated with the development of worsening renal function in elderly patients admitted with heart failure, and to examine the impact of worsening renal function on clinical and economic outcomes. The study sample included 1,681 patients aged ≥65 years, discharged with heart failure at 18 Connecticut hospitals, who did not have clear precipitants for renal dysfunction. Worsening renal function (defined as an increase in serum creatinine level of >0.3 mg/dl during hospitalization from admission) occurred in 28% of the cohort and was associated with male gender, hypertension, rales > basilar, pulse >100 beats/min, systolic blood pressure >200 mm Hg, and admission creatinine >1.5 mg/dl. Based on the number of these factors, a patient’s risk for developing worsening renal function ranged between 16% (≤1 factor) and 53% (≥5 factors). After adjusting for confounding effects, worsening renal function was associated with a significantly longer length of stay by 2.3 days, higher in-hospital cost by $1,758, and an increased risk of in-hospital mortality (odds ratio 2.72; 95% confidence interval 1.62 to 4.58). In conclusion, worsening renal function, an event that frequently occurs in elderly patients hospitalized with heart failure, confers a substantial burden to patients and the healthcare system and can be predicted by 6 admission characteristics.