In the Kiel-classification of non-Hodgkin lymphomas a distinction is made between low and high grade malignancy. Mycosis fungoides is listed among the low grade malignant lymphomas because in the cellular infiltrate small lymphoid cells are predominant. Two cases are reported in which the cellular infiltrate of the cutaneous lesions consists mostly of large lymphoid cells resembling lymphoblasts and immunoblasts indicating a high grade malignancy. This type of cellular infiltrate appeared as a blastic transformation from mycosis fungoides very early in one case and at a later stage in another. Clinically these two cases were characterized by a rapid progression leading to the deaths of the patients. The cytological analysis in cutaneous lesions of mycosis fungoides seems to have a prognostical and possibly a therapeutical bearing.