We examined whether worsening renal function (RF) was associated with long-term mortality, readmission, and inpatient costs in Medicare beneficiaries hospitalized with heart failure (HF). Baseline renal insufficiency in patients hospitalized for HF is associated with increased risk of morbidity and mortality. However, the relation between worsening RF and long-term clinical outcomes is unclear. We linked clinical registry data to Medicare inpatient claims to identify 1-year outcomes of patients > or =65 years of age hospitalized with HF. Worsening RF was defined as a change in serum creatinine > or =0.3 mg/dl. Relations between worsening RF and 1-year mortality and readmission were evaluated with multivariable Cox proportional hazards models with robust SEs; associations with inpatient costs were evaluated with generalized linear models with a log-link and Poisson distribution. Of 20,063 patients hospitalized with HF and discharged alive, 3,581 (17.8%) had worsening RF during the index hospitalization. One year after discharge, 35.4% of these patients died, 64.5% were readmitted, and average costs at 1 year were $14,829 (interquartile range 0 to 19,366). After adjustment for patient characteristics, baseline RF, and comorbid conditions, worsening RF was independently associated with 1-year mortality (hazard ratio 1.12, 95% confidence interval 1.04 to 1.20) but not readmission or total inpatient costs. In conclusion, worsening RF in patients hospitalized with HF was independently associated with long-term mortality.