Interdigitating dendritic cell sarcoma is an extremely rare neoplasm derived from professional antigen presenting cells. We report an unusual case of such a tumor occurring in a 61-year-old woman who had undergone orthotopic liver transplantation for stage IVA2 primary hepatocellular carcinoma with a raised preoperative α-fetoprotein level, followed by tacrolimus-based immunosuppressive therapy. During her subsequent management, the tacrolimus blood levels ranged from 7.9 ng/mL to 16.1 ng/mL. Physical examination revealed bilateral neck and left axillary lymphadenopathy. No evidence of either chronic hepatitis B virus or Epstein-Barr virus could be detected in serum. An excisional biopsy of a right neck lymph node was performed. Microscopically, the normal architecture was diffusely effaced by a proliferation of spindled to ovoid cells arrayed in a fascicular, ill-defined whorled pattern and small lymphocytes were admixed in varying numbers with the tumor cells. Immunohistochemical studies showed that the tumor cells were positive for S100 protein, vimentin and CD68. Based on these findings, the case was diagnosed as an interdigitating dendritic cell sarcoma. The patient unfortunately had no response to 2 cycles of CHOP chemotherapy (cyclophosphamide, doxorubicin, vincristine, and prednisone), and died of wide spread disease 6 months after the original biopsy. We propose that tacrolimus-based immunosuppression was associated with the development of interdigitating dendritic cell sarcoma after liver transplantation in this case.