Hodgkin's disease and mycosis fungoides have been rarely reported in the same patient. This coexistence has been debated in the medical literature. We studied such a patient and report, to our knowledge, the first immunophenotypic evidence for such a coexistence. Reed-Sternberg cells and their variants stained with anti-Leu-M1, Hefi-1, anti-Tac, anti-HLA-DR, and OKT9, but were negative for T cell markers 3A1, Leu-1, Leu-2a, and Leu-3a, a phenotype typical of Hodgkin's disease; infiltrating small lymphocytes were predominantly T cells and were phenotypically normal. In the skin lesions, cells with the phenotype of Hodgkin's disease were not present; the infiltrate was composed of helper T lymphocytes that were 3A1-negative, a phenotype characteristic of the malignant cells of mycosis fungoides. Unexpectedly, a dermatopathic lymph node from the same patient showed the presence of the Leu-M1 antigen on the majority of normal-appearing interdigitating reticulum cells; this was not the case with control dermatopathic lymph nodes from patients without a malignancy. The significance, implications, and possible interrelationships of the findings are discussed.