In the last few years, postmeniscectomy osteonecrosis has been reported, especially in elderly patients, with low incidence, unclear physiopathology, and without long periods of follow-up. We report a case diagnosed and followed-up for a 3-year period using magnetic resonance imaging. In the initial phase, a large area of intramedullary edema was evident; after 3 months, the edema had decreased (it was no longer evident at 1 year examination) and a clearly defined area of osteonecrosis was evident with very high signal intensity on high-contrast sequences. After 3 years, the lesion showed a degenerative cyst appearance with an osteosclerotic rim and completely disrupted cartilage.