BACKGROUND: The Chinese herbal dietary supplements Chaso and Onshido are marketed for weight loss in Japan. The safety of these weight loss aids is unknown. OBJECTIVE: To describe patients who developed liver injury while taking Chaso or Onshido. DESIGN: Case series. SETTING: Keio University Hospital, Tokyo, Japan, and other hospitals in Japan. PATIENTS: 6 patients who took Chaso and 6 patients who took Onshido before presenting with liver injury. MEASUREMENTS: Pathologic, clinical, and laboratory evaluations and chemical analysis of the herbal weight loss aids. RESULTS: All 12 patients developed acute liver injury characterized by a marked increase in serum liver chemistry values (mean alanine aminotransferase level, 1978 U/L [range, 283 to 4074 U/L]) after ingesting these products. Two patients developed fulminant hepatic failure: 1 patient required liver transplantation, and the other patient died. N-nitroso-fenfluramine, a variant of the appetite-depressant drug fenfluramine, was present in these products. CONCLUSIONS: The use of the weight loss aids Chaso and Onshido may be associated with acute liver injury. N-nitroso-fenfluramine is a possible hepatotoxic ingredient.