We report a patient with mycosis fungoides of 20 years standing who developed mixed cellularity Hodgkin's disease. Full investigation, including biopsy, is essential when recurrent mycosis fungoides is suspected to outrule a second lymphoma as the results may affect management. Case History A Caucasian male presented aged 41 years with an eczematous rash affecting his trunk and upper and lower limbs. There was no lymphadenopathy/organomegaly or mucosal disease. Biopsy showed mucosis fungoides. This was controlled over the next two decades with simple emulsifying creams and topical corticosteroids, the disease remaining confined to the skin. Nineteen years after the diagnosis of mucosis fungoides, he developed an isolated left groin node, biopsy of which showed mixed cellularity Hodgkin's disease. Staging investigations were undertaken and the patient was found to have stage 1A disease (Ann Arbor). He was treated with combination chemotherapy (Nitrogen Mustard, Vincristine, Procarbazine and Prednisolone) and has had no recurrence of his Hodgkin's lymphoma, follow-up being seven years. His mycosis fungoides skin lesions improved temporarily with the cytotoxic therapy, but have subsequently progressed to the tumorous stage. Only temporary improvements in these lesions have resulted from total skin electron therapy, local electron irradiation and P.U.V.A.