Brown tumor is not a true tumor, being an unusual reactive lesion in association with primary or secondary hyperparathyroidism. We report a 23-year-old woman, who initially presented with lower back pain caused by ureterolithiasis. The initial diagnosis of brown tumor was delayed, but later pain in her leg worsened and a sacral lesion was incidentally discovered on lumbar magnetic resonance imaging (MRI); multiple destructive bone lesions were then found radiologically. The radiological features of the multiple bone lesions, which mimicked multiple metastatic tumors, seemed to be those of the terminal stage of malignancy. However, pathological examination and abnormal laboratory data showing elevated serum calcium, alkaline phosphatase, and parathyroid hormone and low serum phosphate confirmed the diagnosis of brown tumor. Adenoma in the parathyroid gland was confirmed and surgically resected. The clinical symptoms of bone pain, and abnormal radiological findings and laboratory data were resolved 6 months after surgery. Synthetic analysis of the clinical, radiological, and laboratory findings was necessary for the definite diagnosis of brown tumor.