Background: The incidence of gastro-intestinal stromal tumors is 1-2% of all gastrointestinal malignancies 1. These rarely metastasize to the heart 2, causing atrial fibrillation. The drug of choice is imatinib. This is known to have GI side effects, and infrequently, direct cardiac toxicity. Here, we present a case of atrial fibrillation in a patient with GIST on imatinib therapy, and outline the workup of this rare presentation.  Case: 84 year old male with spindle cell GIST (4.9 X 3.5 cm) in the gastric fundus, on imatinib 400mg OD for 3 weeks was hospitalized for new onset atrial fibrillation (Figure 1), fever and lymphadenopathy. No history of shortness of breath, chest pain, headaches or palpitations. There is history of severe diarrhea, vomiting and weakness in the last week.  On examination, vitals were normal. Bloodwork showed an eosinophil manual count of 73% (normal: <7%) and atypical lymphocytosis. Workup for the cause of atrial fibrillation (echo, cardiac CT, chest X ray, electrolytes) was done and was normal. Patient was started on eliquis 2.5 mg BD and rate control for atrial fibrillation. Patient developed AKI and transaminitis during the course of hospitalization. Imatinib was held, and patient was started on steroids. On steroids, his eosinophil count normalized, rhythm returned to sinus, renal and liver function dropped back to baseline. The patient was referred to oncology to switch to a different tyrosine kinase inhibitor. Steroids were tapered and discontinued. Patient was asked to review in cardiology clinic for follow – up of atrial fibrillation.  Discussion: Four potential causes of AF were considered in this patient: cardiac metastasis from the GIST, direct drug toxicity, dehydration induced, and Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) syndrome due to imatinib 3,4. Imaging revealed no evidence of cardiac metastasis. Electrolytes were normal, ruling out dehydration as a cause. AF is an uncommon side effect of imatinib (0.55% of patients) 5. In this case, temporal correlation suggested a possible association. However, the markedly elevated eosinophil count raised suspicion for DRESS 6. Scoring according to RegiSCAR system suggested probable DRESS. The patient markedly improved on steroids, further confirming our theory. Several studies have reported cases of imatinib-induced DRESS syndrome, and atrial fibrillation in DRESS (7.1%) 3 supporting our clinical findings.  In conclusion, this case is a good example of the importance of considering multiple etiologies for AF in cancer patients undergoing targeted therapy. Clinicians should remain vigilant, follow a comprehensive diagnostic approach and promptly intervene to mitigate adverse events.