Granulomatous slack skin (GSS) is a rare form of cutaneous T-cell lymphoma, closely related to mycosis fungoides. It is characterized by pendulous skin folds with a predilection for flexural areas. Histology shows an elastolytic granulomatous infiltrate with atypical lymphoid cells. Granulomatous mycosis fungoides is an important histologic differential diagnosis to be considered. We present a 19-year-old man with a gradually enlarging erythematous, and bulky lesions on his body. Histologically, a dense atypical lymphoid cell infiltration with numerous multinucleated giant cells and elastolysis was observed. T-cell receptor gene rearrangement was detected in skin lesions. He was treated with PUVA and interferon α, but improvement in skin lesions was not observed. Marked regression of all lesions was achieved by using electron beam therapy. This case report supports that GSS is an indolent variant of mycosis fungoides due to clinical, histological and T-cell gene rearrangement results. However, there is no definitive data about prognosis of the disease. We suggest that further clinical studies are needed to understand this rare condition.