OBJECTIVE: We studied prevalence, causes and consequences of worsening renal function (WRF) during hospitalization for acute heart failure (AHF). METHODS: Patients admitted for AHF were enrolled. Patients with severe chronic renal failure, cardiogenic shock and contrast medium-induced nephropathy were excluded. WRF was defined as an increase of 25 mumol/l or more in serum creatinine relative to the admission level. Survivors were monitored for 6 months, focusing on deaths and first unscheduled readmissions for heart failure. RESULTS: Among the included 416 patients, WRF occurred in 152 cases (37%), 5+/-3 days after admission, and two-thirds of patients recovered their baseline renal function before discharge. Old age, diabetes, hypertension and acute coronary syndromes increased the risk of WRF. In-hospital furosemide doses as well as discharge treatment were similar in WRF and no-WRF patients. Serum creatinine elevation was the strongest independent determinant of a longer hospital stay (r=0.37, p=0.001). Adverse events occurred in 158 patients (38%) during follow-up, with 23 deaths and 135 readmissions. Cox analysis showed that WRF, transient or not, was an independent predictor of the risk of death or readmission (hazard ratio=1.74 [1.14-2.68], p=0.01). CONCLUSION: WRF is frequent after admission for AHF and, although transient, is associated with longer hospitalization and with a higher risk of death and readmission, irrespectively of baseline renal function.