Primary hyperparathyroidism is mainly caused by parathyroid adenoma (85%) and is characterized by hypercalcemia, osteoporosis, renal stones, and gastrointestinal and neurological disorders. Because of improvements in blood analysis over the last two decades, primary hyperparathyroidism is typically diagnosed early and asymptomatically. A rare clinical manifestations of primary hyperparathyroidism, brown tumors (osteitis fibrosa cystica), are osteolytic lesions resulting from long-term hyperparathyroidism. Radiologically, it is difficult to distinguish a brown tumor from plasmacytoma, multiple myeloma, or bone metastasis. We report a case of a 44-year-old man with primary hyperparathyroidism that caused a large brown tumor (11 × 5 × 8 cm) that mimicked plasmacytoma or cancer metastasis on pelvic magnetic resonance imaging. After a bone biopsy report that was highly suggestive of a brown tumor, serum calcium and intact parathyroid hormone levels were determined. The lesion was ultimately diagnosed as a brown tumor and a parathyroidectomy was performed. After 1 year, the lesion has nearly regressed by follow up of the anteroposterior view of the pelvis and bone mineral density has improved. The present case highlights the importance of considering brown tumors in the evaluation of patients presenting with hypercalcemia and osteolytic lesions without definite primary neoplasm.