Heart failure often causes acute kidney injury (AKI) and the relationship is called cardiorenal syndrome. Not only low cardiac output, but also renal congestion is increasingly considered as the major cause of AKI among heart failure patients. Renal congestion may be also caused by other pathologic conditions such as liver cirrhosis and hypoalbuminemia. Decongestion by diuretics can improve cardiorenal syndrome, but it is difficult to prove that the kidney rather than the heart was the main site of action. In the present report, a patient developed severe HELLP syndrome immediately after delivery of a baby. Thrombocytopenia and elevated liver enzymes were largely improved within a week, but anuria and severe ascites continued, which were resistant to oral or iv diuretics. We believe this was a unique case with purely mechanical renal congestion, not associated with heart failure or liver dysfunction. N/A. The case was a 27-year-old woman. Ascites appeared during pregnancy and a baby was delivered by emergency caesarean section at 38 weeks. At that time, there was no hypertension and proteinuria. At 3 days after delivery (D3), hypertension (160/90 mmHg), thrombocytopenia (18,000/uL), liver dysfunction (AST 1,928 U/L, ALT 622 U/L, LDH 2,352 U/L), and indirect bilirubin-dominated jaundice developed (TBIL 3.6 mg/dL, DBIL 0.9 mg/dL). HELLP syndrome was diagnosed. Abdominal fullness, edema, punctate hemorrhage, gross hematuria, proteinuria (2.4 g/gCr) were observed, but renal function was normal (serum Cr 0.68 mg/dL). Carperitide (atrial natriuretic peptide), antihypertensive, loop diuretic and tolvaptan were started for hypertension, fluid retention, edema and oliguria. Thrombocytopenia and liver dysfunction gradually improved, but weight gain (62 → 69 kg) and renal function worsening (D11, Cr 6.6 mg/dL) were observed. It seemed difficult to correct fluid excess by drug treatment, and hemodialysis and ultrafiltration were performed for 3 consecutive days on D11-13, resulting in an increase in urine output and improvement of proteinuria (to 0.4 g/gCr). The patient was discharged on D24 at 59 kg, and Cr was improved to 1.0 mg/dL on D28. HELLP syndrome may cause intra organ bleeding and renal failure, potentially resulting in a severe clinical outcome. In this case, although the primary disease seemed to be improving, ascites resulted in oliguria likely due to an increase in the abdominal pressure, and the level of ascites appeared to exceed the limit of natural recovery. Only 3 days of acute renal replacement therapy with ultrafiltration was sufficient to trigger resolution of all the clinical abnormalities such as ascites, oliguria and proteinuria. There were no signs of prerenal, renal parenchymal or post-renal AKI. This was a unique AKI case caused purely by reversible renal congestion, suggesting that renal congestion is the fourth, independent category of AKI.