Introduction: Extraintestinal manifestations (EIMs) of inflammatory bowel disease (IBD) are well-established. Sweet syndrome (SS), or acute febrile neutrophilic dermatosis, is a reactive mucocutaneous manifestation of IBD presenting as erythematous papules/plauqes with associated fever and arthomyalgias. There is limited evidence regarding efficacy of Ustekinumab in management of EIMs of UC, particularly SS. Case Description/Methods: The patient is a 50-year-old woman with left-sided UC (Montreal classification E2) that previously failed several therapies, including mesalamine, azathioprine, infliximab, adalimumab, vedolizumab, and a fecal microbiota transplant trial. She presented with worsening abdominal pain, diarrhea/hematochezia, and bilateral lower extremity pain. She was febrile and tachycardic with tender, violaceous, and ulcerated subcutaneous nodules on the bilateral ankles with hemorrhagic bullae (FigureA). Labs revealed negative C. diff, elevated ESR/CRP, thrombocytosis, and borderline leukocytosis with neutrophilic predominance. Lower extremity MR imaging and bone biopsy revealed multifocal sterile osteomyelitis with adjacent abscess; skin biopsy demonstrated dense granulomatous/neutrophilic infiltrate in the deep dermis/subcutis without microorganisms, consistent with subcutaneous SS (FigureB). A restaging colonoscopy demonstrated severe proctosigmoid ulcerative colitis (FigureC). Given that the patient had failed several immunomodulators and biologic therapies, she was started on prednisone and Ustekinumab. Four months later, she reported complete resolution of her cutaneous SS off steroids and improving UC symptoms (less frequent and more formed stools, minimal hematochezia). Repeat MRI demonstrated resolution of the sterile osteomyelitis and abscess. Discussion: Our case highlights the novel use of Ustekinumab in the treatment of subcutaneous SS with sterile osteomyelitis in a patient with flaring UC. SS is a neutrophilic dermatosis that is a rare EIM of IBD, classically characterized by tender cutaneous lesions. There is limited evidence supporting the use of Ustekinumab in treating EIMs of CD, and a lack of data regarding its efficacy in treating EIMs of UC. Our case fills a major gap in the literature and highlights the clinical efficacy of Ustekinumab for SS and UC especially in patients that have a contraindication to, failed previous treatment with, or otherwise cannot tolerate corticosteroids and TNF-alpha antagonists.Figure 1.: 1A. Right lateral malleolus with 2.5 cm shallow erosion with slightly violaceous borders and central bright yellow fibrinous debris. There was slight atrophy of surrounding skin with mild edema and hyperpigmentation (left image). Left medial malleolus/dorsal foot with grouped irregular shallow ulcers with surrounding mild violaceous erythema (middle and right image). 1B. Left ankle biopsies showed mixed septal and lobular panniculitis with granulomatous and neutrophilic inflammation. Histologic sections show a mildly spongiotic epidermis with dense granulomatous and neutrophilic inflammatory infiltrate in the deep dermis and subcutis (left image). The inflammatory infiltrate, which consists of prominent neutrophils, histiocytes, and scattered lymphocytes, surrounds vascular and adnexal structures, as well as adipocyte lobules. Definitive features of vasculitis are not seen. PASd (periodic acid-Schiff-diastase), GMS (Grocott’s methenamine silver), Gram, and Fite staining fail to highlight microorganisms (right image). 1C. Colonoscopy demonstrated severe (Mayo Endoscopy Score 3) proctosigmoid ulcerative colitis to 30 cm (left image). Sigmoid biopsies showed significant immune cell infiltration with crypt architectural distortion concerning for severe chronic active colitis. No evidence of granulomas, dysplasia or cytomegalovirus (right image).