A 50-year-old patient with knee pain and radiological findings indicating an aggressive lesion. Anteroposterior radiography of the knee, showing tapering of the cortical bone in the proximal tibia, presence of a mottled, mixed medullary lesion with an imprecise zone of transition, extending from the subchondral boné to the diaphysis. The main differential diagnosis is metastasis, hyperparathyroidism, and lymphoma. Laboratory tests confirmed the diagnosis of multiple myeloma.