Sweet syndrome (acute febrile neutrophilic dermatosis) is a relatively rare inflammatory di-sease, which is characterized by the sudden appearance of tender erythematous skin lesions, often accompanied by pyrexia and elevated neutrophil count. The pathogenesis is not clear yet. Recently, multiple studies have found the association between Sweet syndrome and infections, autoimmune diseases, malignant tumors and the application of multiple drugs. According to different causes, Sweet syndrome can be divided into three types: classical (or idiopathic) Sweet syndrome, malignancy-associated Sweet syndrome and drug-induced Sweet syndrome. Classical Sweet syndrome usually presents in women between the age of 30 to 50 years and may be related to infection, inflammatory bowel disease, or pregnancy. The clinical symptoms typically respond promptly after corticosteroid therapy. The major diagnostic criteria of classical Sweet syndrome include sudden painful erythematous skin lesions, histopathologic evidence of a dense neutrophilic infiltrate without evidence of leukocytoclastic vasculitis; minor criteria include pyrexia over 38 ℃, association with hematologic or visceral malignancies, inflammatory diseases, pregnancy or preceded by infection, prompt response to systemic glucocorticoid or potassium iodide treatment, abnormal laboratory values (three of four: erythrocyte sedimentation rate >20 mm/h, positive C-reactive protein, >8.0×10⁹/L leukocytes, >70% neutrophils). The presence of both major criteria and two of the four minor criteria are required to diagnose classical Sweet syndrome. As for the malignancy-associated Sweet syndrome, skin lesions can be found precede, follow, or at the same time with the diagnosis of hematologic malignancy or a solid tumor. At present, ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) is commonly used as a positron emission tomography computed tomography (PET/CT) imaging agent for diagnosing and screening malignant tumors. Therefore, most of the case reports on the ¹⁸F-FDG PET/CT manifestations of Sweet syndrome are malignancy-associated. Even classical Sweet syndrome is often accompanied by inflammatory bowel disease, autoimmune diseases, etc. Therefore, for patients with suspected or confirmed Sweet syndrome, it is necessary to take ¹⁸F-FDG PET/CT examination to clarify the general condition, whether it is for patients with malignant signs such as elevated tumor markers values and weight loss, or for patients with classical Sweet syndrome to exclude underlying inflammatory diseases. ¹⁸F-FDG PET/CT is often able to detect the solid tumor early, and assess the degree of hematologic malignancy and inflammatory disease. This study reported a classical Sweet syndrome case associated with inflammatory bowel disease, which was confirmed with skin and intestinal histological examination. The clinical manifestations, laboratory values, ¹⁸F-FDG PET/CT manifestations of the patient related diseases were reported, which was to improve nuclear medicine physicians' understanding of Sweet syndrome. Early diagnosis and treatment can often achieve excellent clinical effect.