New knowledge of the function and biological behaviour of the cells of the lymphocytic system justifies a critical reappraisal of the nature and origin of mycosis fungoides (MF). The fact that in MF the immune response remains active during the whole course of the disease distinguishes it from other malignant processes. On the basis of the well-known fact that chronic stimulation of the immunological system may lead to tumour formation, the hypothesis is stressed that MF may be an ‘immunocytoma’ or ‘immunoma’. New findings indicate that immunological stimulation of lymphocytes may lead to the formation of endogenous oncogenic viruses. All these questions should be investigated in as many centres as p