The prognosis of mycosis fungoides and Sézary syndrome can be improved when antitumor therapies such as total skin electron beam irradiation, topical nitrogen mustard, or systemic cytostatic agents are given as soon as the diagnosis is established. However, differentiation of early lesions of mycosis fungoides and Sézary syndrome from chronic benign dermatoses remains very difficult. To aid in the differential diagnosis, more objective techniques have been developed, including DNA cytophotometry, chromosome analysis, quantitative electron microscopy, and monoclonal antibody staining. Using these methods, skin infiltrates and lymph nodes can be classified more precisely as malignant or benign at an earlier stage of the disease. It must be stressed that these methods can be used only in conjunction with other clinical and histomorphologic criteria. They should never be used alone, however, because reactive processes may sometimes cause problems in interpreting the results.