OBJECTIVES: This study assessed the impact of worsening renal function (WRF) during hospitalization on healthcare resource utilization in hospitalized heart failure (HF) patients. METHODS: A retrospective, longitudinal study using patient-level data provided by Geisinger Health System MedMining database that captures clinical and cost data for inpatient and outpatient episodes, inpatient medication use and pharmacy claims. It included adult hospitalized patients with HF as a primary diagnosis defined via relevant ICD-9 code(s).  Patients were excluded if they had no previous HF, or were admitted for acute coronary syndromes or cardiac surgery at or within 30 days prior to the first HF-related hospitalization (index hospitalization).  WRF was defined as ≥0.3 mg/dL increase in serum creatinine (SCr) at any time during hospitalization.  Univariate analyses were used to assess the association between WRF and healthcare resource utilization.  A priori statistical significance level of 0.05 was used for these analyses. RESULTS: Of 5803 hospitalized patients with primary HF diagnosis, 827 (14%) patients fulfilled all pre-specified inclusion and exclusion criteria.  Mean age (±SD) was 73.5±14.1 years, 43% were male, 98% Caucasians and ≥80% had some renal impairment at admission based on MDRD eGFR <90ml/min/1.73m2.  Average admission SCr was 1.4±0.9 mg/dL. Nearly 33% of patients exhibited WRF during index hospitalization.  As compared to hospitalized HF patients w/o WRF, those with WRF had greater prevalence of diabetes (54% vs. 43%), reduced baseline eGFR (43.7±29.7 vs. 62.2±35.3 ml/min/1.73m2), elevated potassium levels (4.3±0.7 vs.4.2±0.7 mEq/L), greater BNP at baseline (844.7±820.8 vs. 794.9±947.2 pg/ml), higher total inpatient cost including medications, physician and hospital services, labs, procedures ($20,829±$26,686 vs. $13,445±$20,392, p<0.001) and greater length of hospital stay (8.2±6.8 vs. 5.7±5.5 days, P<0.001).  CONCLUSIONS: Among hospitalized HF patients, occurrence of WRF was associated with an increase in the hospitalization cost by 58% and length of stay by 41% versus absence of WRF.