Langerhans cell histiocytosis (LCH), formerly called “eosinophilic granuloma,” is a benign bone neoplasm that is rare in adults. It is a cellular lesion with histiocytes, plasma cells, small lymphocytes, neutrophils, giant cells, foam cells, and eosinophilic leukocytes1. The disease typically presents during childhood, with >90% of cases occurring between the ages of five and fifteen years. LCH typically presents as a lytic medullary lesion with sharply defined margins, most commonly in the skull and in the diaphyses of long bones. The pelvis is affected in 5% to 17% of reported cases2,3. We report an unusual case of LCH presenting in the femur of a healthy young adult man following primary total hip arthroplasty for presumed idiopathic osteonecrosis. Inspection of the joint at the time of the initial surgery demonstrated no findings that were thought to warrant formal histological investigation of bone at the arthroplasty site. One year later, repeat radiographs demonstrated changes mimicking a periprosthetic infection. The prosthesis was removed, and histological examination of tissue curetted from the lytic lesions adjacent to the prosthetic femoral stem demonstrated findings consistent with a diagnosis of LCH. Cultures of curetted tissue from the same site revealed no bacterial, mycobacterial, or fungal growth. The patient, treated initially as if he had an infection at the site of the arthroplasty, underwent placement of an antibiotic cement spacer followed by revision total hip arthroplasty. At the most recent follow-up evaluation, there had been no recurrence of the LCH and the hip prosthesis was functioning well. In retrospect, LCH, which was present at revision, could have been present at the primary operation and could have been diagnosed if tissue specimens had been submitted for histological examination. This experience underscores the importance of maintaining a high index of suspicion for unexpected diagnoses …