Atopic dermatitis (AD) is not regarded as a predisposing condition for cutaneous T cell lymphoma. Cyclosporine (CsA) is an efficient therapy in AD, and its side effects, including lymphocytic proliferation, are rare at the low dose used in such cases. So far 3 cases of patients who developed cutaneous T cell lymphoma under CsA treatment for atopy have been described. An adult patient with a history of AD received CsA therapy because of an atypical flare-up of his eruption. He rapidly developed papular atypical lesions, then followed by the onset of a voluminous cutaneous tumor leading to the diagnosis of mycosis fungoides (MF) transformed in CD30+ large cell lymphoma with nodal and blood involvement. No improvement occurred after withdrawing of CsA, and he required a polychemotherapy and an allograft. This case illustrates the dramatic evolution of an MF under CsA given in the hypothesis of AD. It stresses the fact that skin biopsies should be taken in case of atypical AD to rule out MF and avoid such an aggressive evolution under immunosuppressive therapy.