We report on the usefulness of the ImmuKnow assay in a case of suspected acute cellular rejection after liver transplantation. A 58-year-old male with hepatocellular carcinoma and liver cirrhosis caused by chronic hepatitis C had undergone tumorectomy 2 months previously. Following surgery, the underlying cirrhosis and hepatic encephalopathy were aggravated. The patient had been listed for liver transplantation and underwent cadaveric donor liver transplantation. Approximately 11 days after discharge, the patient developed mild fever and diarrhea and was rehospitalized. A liver biopsy showed histologic features associated with cellular rejection. According to the histopathologic diagnosis, the dosage of tacrolimus was increased from 5 to 7 mg twice daily. After changing the dose, aspartate aminotransferase and alanine aminotransferase were elevated, findings not corresponding to the former diagnosis. Hepatitis C virus (HCV) quantitative assay and ImmuKnow assay were performed for further evaluation. High HCV viral load and a very low ATP level detected using the ImmuKnow assay were suggestive of recurrent HCV rather than acute cellular rejection. Two weeks after reducing the immunosuppressant dosage and treating with antiviral therapy using ribavirin, the patient showed clinical improvement with a decrease in HCV viral load and a normal ATP level. Due to overlapping histologic features, acute cellular rejection can be difficult to distinguish from recurrent HCV. As in this case, the ATP level detected using the ImmuKnow assay is considered a reliable marker of cellular immune status. Immune monitoring of transplant patients may assist in making a differential diagnosis and in minimizing the adverse events of immunosuppression.