BACKGROUND Erdheim-Chester disease (ECD) is a rare neoplasm of histiocytes that is characterized by prominent involvement of the long bones. Approximately 1500 cases have been reported since the disease was first described in 1930. The imaging appearance of ECD can be highly variable given the numerous systems it can affect. In this case report we discuss a patient whose ECD was occult on multiple imaging modalities. CASE REPORT We report the case of a 60-year-old woman who presented with sub-acute left knee and calf pain that led to an MRI. She was found to have innumerable marrow-replacing lesions in the axial and appendicular skeleton visualized on the initial MRI, as well as on an ¹⁸F-FDG PET/CT scan. The patient did not have extraosseous abnormal uptake on the PET/CT. Subsequently, a lesion from the left iliac bone was histologically confirmed as ECD on the basis of positive staining for CD68 and CD163 and negative staining for CD1a. Osseous lesions in ECD have a distinct imaging appearance and are typically detected by radiography and bone scintigraphy, among other modalities; however, the lesions in this case were unexpectedly absent from those studies. CONCLUSIONS If there is a high degree of suspicion for ECD, 18F-FDG PET/CT and/or MRI may be necessary for adequate visualization of bone lesions, given that those lesions can have an infiltrative nature that may be difficult to image with other anatomic imaging modalities. Use of 18F-FDG PET/CT and/or MRI may also lead to adequate guidance of confirmatory biopsy.