Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare disease. The prognosis is poor in most cases with rapid progression despite administering chemotherapy. A 67-year-old man complained of skin rashes on his back and this spread to the trunk, face, arms and thighs, and he was initially diagnosed with cutaneous lupus erythematosus according to the skin biopsy. The skin rashes then became aggravated on a trial of low dose methylprednisolone for 3 months. Repeated skin biopsy revealed a diffuse infiltration of lymphoid cells with medium sized nuclei, positive for CD4 and CD56, negative for Epstein-Barr virus (EBV), indicating a diagnosis of BPDCN. Further workups confirmed stage IVA BPDCN involving the skin, multiple lymph nodes, the peripheral blood and the bone marrow. He was treated with six cycles of combination chemotherapy consisting of ifosphamide, methotrexate, etoposide, prednisolone and L-asparaginase, and he achieved a partial response. Herein we report on a rare case of BPDCN that was initially misinterpreted as cutaneous lupus erythematosus.