We present the case of a 65-year-old man with brown tumors due to secondary hyperparathyroidism. Magnetic resonance imaging of the pelvis showed multiple lesions with expansive bone appearance. Additionally, prostate cancer was diagnosed during this time. For this reason, differential diagnosis was performed through biopsy of the right iliac bone lesion. Brown tumors are caused by osteoclastic activity and fibroblast proliferation;​ the differential diagnosis of these bone lesions includes giant tumors, metastases, Paget’s disease, and paraneoplastic syndrome with high levels of parathyroid hormone-related peptide (PTHrP). This case report describes the coexistence of two pathologies that could explain these images. In this report, we present a case of a 65-year-old man with brown tumors due to secondary hyperparathyroidism and prostate cancer. In this setting, histologic confirmation is recommended.