Total skin irradiation with fast electrons (4 MeV, 3,500 rad) was applied in 10 cases of histologically proven mycosis fungoides without any signs of internal dissemination. The technique is described. Complete disappearance of both lesions and symptoms occurred in all patients after treatment. However, half of the patients relapsed after the initial irradiation and were subsequently treated with a booster dose of 1,000 rad 4 MeV electrons or with topical nitrogen mustard. Irradiation with fast electrons seems to be of great importance in the treatment of mycosis fungoides.