the clonicity detection in the lymphoproliferative syndromes by studying the rearrangement of the immunoglobulin genes and of the t-receptor cells is used to make clear if a proliferation or infiltrate of lymphocytes is malignant or not. this type of study is particularly useful in the presence of cutaneous lesions whose lymphoid or dermatological origin is difficult to define. by the pcr-heteroduplex technique, the genes of the immunoglobulin heavy chain and of the t-cell receptor chain were studied in 10 patients that presented dermatological manifestations attributable to some kind of cutaneous lymphoma. clonal rearrangement was observed in 7 patients, which allowed to confirm the diagnosis of mycosis fungoides and other types of cutaneous lymphomas. it was not possible to confirm a lymphoid process of malignant character by this technique in 3 patients who did not show clonal rearrangement. the usefulness of the study was proved when in the presence of a skin affection, it was difficult to differentiate a dermatological process from a proliferative syndrome with cutaneous manifestations.