BACKGRAOUND: Langerhans cell histiocytosis is a rare hematological disorder. Skin rash is the typical early feature, and bony involvement is the second most common presentation.METHODS: We present a case of a 5-month-old female infant with left hemifacial swelling, initially treated for infection with antibiotics. However, due to persistence of swelling and new onset fever, further evaluation with ultrasonography, CT scan, FDG PET/CT and eventually biopsy was performed. RESULTS: Imaging methods revealed mandibular osteolysis indicative of either osteomyelitis or histiocytosis X. Tissue biopsy was diagnostic for Langerhans cell histiocytosis.CONCLUSION: Langerhans cell histiocytosis may present in infancy with a variety of symptoms, included an isolated bony lesion. Langerhans cell histiocytosis, despite its rarity, should be included in the differentiated diagnosis, when bone osteolysis is found.KEYWORDS: Langerhans cell histiocytosis, osteolysis, PET, CT, imaging