Langerhan's histiocytosis is a not uncommon disease, but a related disorder, Rosai-Dorfman (RDD) with extensive or exclusive extranodal distribution, is relatively rare. Ga and PET/CT imaging have typically been used for diagnosis and tracking response to treatment. The authors present a very unusual case wherein lesions of RDD actively accumulated Tc-sestamibi, which was injected as part of a scan to localize a parathyroid adenoma.