A 58-year-old female patient presented with several weeks history of significant bilateral knee pain. Initial knee radiographs demonstrated lucencies of the bony cortex while extensive osteolytic lesions on a routine chest radiograph were suggestive of multiple myeloma or bony metastases. Biochemical investigation revealed primary hyperparathyroidism with renal insufficiency. A parathyroid adenoma was demonstrated on a neck ultrasound and sestamibi scan and subsequently confirmed by histology. We illustrate a case of primary hyperparathyroidism with osteitis fibrosa cystica and brown tumours which were initially mistaken for malignant disease.