Worldwide use of herbal therapy has increased dramatically in recent years. Most herbal therapies were not regulated as medicines, and their adverse effects often were underreported. We report a patient who developed acute renal failure, acute hepatic failure, autoimmune hemolytic anemia, and thrombocytopenia after oral intake of hot-water extract of Cupressus funebris Endl (Mourning Cypress), which is rich in flavonoids. Her renal biopsy showed acute tubular necrosis, interstitial nephritis, and hemoglobin casts. The clinical course and pathological findings were consistent with flavonoid-induced acute nephropathy. We emphasize that flavonoids are not harmless and may induce acute life-threatening renal damage.