Granulomatous slack skin (GSS) is an extremely rare subtype of cutaneous T-cell lymphoma. A 14-year-old boy had suffered from progressive infiltrative erythema and plaques that gradually evolved into lax masses and pendulous skin on his axilla, anterior wall of the abdomen, bilateral inguinal region, and thighs. Histopathologic examination of the skin lesion and inguinal lymph node demonstrated granulomatous infiltration with multinucleated giant cells. Positron emission tomography (PET)/computed tomography (CT) scan was performed after acute exacerbation and exhibited slightly high fluorodeoxyglucose (FDG) distribution of skin lesions, without any evidence of abnormality in the metabolism of FDG in lymph nodes or other extralymphatic organs. Concurrent use of corticosteroid and recombinant interferon-alpha successfully controlled the disease, and posttreatment PET/CT scan confirmed the response to the therapy with decreased levels of FDG uptake. PET/CT is suggested to be helpful in the assessment of disease progression and treatment response in the management of patients with GSS.