The case of a 69-year-old male patient with an unusual type of malignant lymphoma is presented. Clinically, it was at first characterized by follicular papules and erythematous patches, later, by the development of cutaneous tumors and enlarged lymph nodes, and by a severe, finally excruciating pruritus. Treatment with PUVA (psoralen-ultraviolet-A) combined with 40--80 mg prednisolone and then with chemotherapy [COPP regimen (cyclophosphamide, vincristine, procarbacine, prednisone), high-dosage methotrexate followed by citrovorum factor rescue] was not successful. The patient died of pneumonia 2.5 years after the onset of the first clinical symptoms. An immunoblastic infiltrate was observed histologically and electromicroscopically in the initial lesions of the skin. Therefore, the diagnosis of a cutaneous immunoblastic T-cell lymphoma was tentatively made at the beginning, which was later confirmed in numerous biopsies and laboratory investigations. Immunocytologically and enzymecytochemically, the infiltrating cells were shown to be immature T cells; in the lymph nodes, numerous immunoblasts and large Sézary cells was observed in the peripheral blood, though there were no very large Sézary cells or blast cells. In the autopsy, a systemic involvement with an atypical lymphoid infiltration was found in numerous internal organs. The special nature of this case justifies its classification as high-grade malignant lymphoma and its differentiation from normal cases of mycosis fungoides. In contrast, mycosis fungoides generally fulfils criteria typical of low-grade malignant lymphomas.