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Word count: 1329Tables:NoneFigure: 3References: 9Conflicts of interest: NoneAcknowledgements: NoneAuthor Contributions: All authors contributed to designing and conducting the work, drafting, and revising the manuscript and approved the final version for submission.ABSTRACTBACKGROUND: We present a case of sirolimus-induced DRESS syndrome in a stem cell transplant patient. Sirolimus is an immunosuppressive drug that inhibits the mTOR pathway. It is commonly used in organ transplants to prevent rejection. While no sirolimus-induced DRESS cases have been reported, allergic reactions with everolimus, a similar drug, have been documented. DRESS syndrome is a severe drug reaction characterized by fever, rash, and organ involvement. Diagnosis is based on clinical findings and laboratory tests. Early recognition, discontinuation of the drug, and supportive care are crucial in managing DRESS syndrome, often involving systemic corticosteroids.CASE REPORT A 24-year-old man who had undergone haplo-TESE transplantation for acute lymphatic leukaemia presented with diffuse itchy eczematous lesions. Initially diagnosed as atopic dermatitis, he received topical steroid therapy and NB-UVB phototherapy, but his condition worsened. Two months later, he returned to the emergency department with eczematous patches, xerosis, fever, chills, and generalized edema. His medical history included relapses of leukaemia, acute cutaneous graft-versus-host disease (GVHD), and Evans syndrome. He had been on sirolimus immunosuppressive therapy before the onset of symptoms. A skin biopsy revealed spongiotic dermatitis with dermal eosinophils, suggestive of drug reaction or atopic reaction. Based on the severity of the symptoms and histological findings, the patient was diagnosed with sirolimus-induced DRESS syndrome. Sirolimus was discontinued, and oral steroid therapy was initiated, leading to significant improvement. At the one-month follow-up, the patient was symptom-free and had lost the gained weight.CONCLUSION Although no cases of sirolimus-induced DRESS syndrome have been reported, allergic reactions with eosinophilia induced by everolimus have been documented. And since sirolimus and everolimus, both mTOR inhibitors, share a common mechanism of action, therapeutic indications, pharmacokinetics, adverse effects and drug interactions, it cannot be ruled out that sirolimus may trigger DRESS syndrome in patients with risk factors. In our case, the