Osteonecrosis is a pathological process which results in death of bone and bone marrow elements and can be associated with a variety of medical conditions. Involvement of the knee is believed to account for approximately 10% of all cases. The most frequent type of knee osteonecrosis is idiopathic osteonecrosis (spontaneous), and commonly involves the medial femoral condyl. Spontaneous osteonecrosis is more common in patients older than 60 years. In this case report, a 31-year-old female patient diagnosed as having spontaneous osteonecrosis of the knee after a sudden pain was presented. Turk J Phys Med Rehab 2010;56:40-2.