INTRODUCTION: Nivolumab, a monoclonal antibody immune checkpoint inhibitor (ICI) acting on PD-1, is an effective treatment for many malignancies The augmentation of T cell immunity leads to its therapeutic effect, yet commonly causes immune-related adverse effects (irAE) ICI-induced diarrhea is frequently reported, while other gastrointestinal irAE are less well described Here, we report a case of nivolumab-induced eosinophilia, ascites, and abdominal pain CASE DESCRIPTION/METHODS: A 58-year-old man with a history of tobacco use and stage IIB lung adenocarcinoma status post partial lobectomy, adjuvant chemotherapy (platinum/pemetrexed), and nivolumab was admitted with 1 month of diffuse abdominal pain and distention, dyspnea, fatigue, and intermittent low-grade fevers He had received 11 cycles of nivolumab over 9 months, but it was stopped 3 months before admission due to eosinophilia On admission, temperature was 99 4 and he had mild sinus tachycardia Initial labs were notable for WBC 22 6 (3 5% bands), absolute eosinophil count 1605 (peak 4563), and platelets 800 CT chest/abdomen/pelvis revealed hepatomegaly, new ascites, and bilateral pleural effusions Paracentesis indicated inflammatory ascites (serum ascites albumin gradient 0 5) Infectious work-up was negative, including COVID-19 testing ECHO showed normal cardiac function and small pericardial effusion He received empiric antibiotics without improvement EGD revealed mild diffuse duodenal lymphangiectasia Duodenal biopsies showed patchy villous blunting with inflammatory infiltrate, including eosinophils within the lamina propria After extensive work-up and multidisciplinary consultation, nivolumab-induced eosinophilia with serositis was diagnosed Empiric high dose steroids (solumedrol) and ivermectin/ albendazole were given, though parasitic infection deemed unlikely After symptoms improved, he was discharged on a prednisone taper DISCUSSION: In this patient presenting with a constellation of symptoms, nivolumab-associated eosinophilia with serositis was the unifying diagnosis, causing the atypical GI symptoms of new ascites and abdominal pain While rare, eosinophilia has been reported with ICI use (1) Thus, irAE must be considered in a patient with new GI symptoms after ICI therapy, as treatment with steroids and/or biologics may be indicated (Figure Presented)