Background <p>Febrile ulceronecrotic Mucha–Habermann Disease (FUMHD) is a rare and severe form of pityriasis lichenoides et varioliformis acuta. It presents rapidly with ulcerative and necrotic lesions, fever, and systemic symptoms. This condition is associated with significant morbidity and can be extremely dangerous. The exact cause of FUMHD is not well understood, but hypersensitivity reactions may trigger it.</p> Case presentation <p>We describe a 15-month-old Iranian boy who developed rapidly progressing ulceronecrotic skin lesions accompanied by high-grade fever and systemic involvement. A skin biopsy confirmed a diagnosis of FUMHD. Despite treatment with methylprednisolone, intravenous immunoglobulin, and a TNF-α inhibitor, the disease was fatal. To our knowledge, this represents the third reported fatal case of FUMHD in children and the youngest case documented to date.</p> Conclusion <p>This report presents the youngest known pediatric case of fatal FUMHD, highlighting the challenges of early diagnosis, risk of systemic infection, and limited treatment options. Early immunosuppressive and antimicrobial therapies are crucial, but prognosis remains poor in aggressive cases.</p>