AIMS: (1) To assess the clonality of tumour cells in two patients with mycosis fungoides who subsequently developed Hodgkin's disease; and (2) to determine whether there is a clonal relation between these two disorders. METHODS: Cutaneous tissue samples involved by mycosis fungoides and lymph nodes involved by Hodgkin's disease from both patients were investigated by immunohistochemistry and the polymerase chain reaction. RESULTS: Mycosis fungoides tumour cells in both patients expressed multiple T cell associated antigens; Reed-Sternberg (RS) cells had the null phenotype. T cell receptor gamma chain genes were clonally rearranged in mycosis fungoides cells but not in RS cells, including variants, in both patients. In the patient with intermediate transformation to large cell lymphoma, immunoglobulin heavy chain genes were rearranged in the cutaneous tumour, but not in the lymph node involved by Hodgkin's disease. CONCLUSION: The divergent antigen expression and gene rearrangements observed in these two patients strongly suggest that Hodgkin's disease and mycosis fungoides are not derived from a single tumour cell clone.