Spontaneous osteonecrosis of the knee (SONK) is generally associated with a poor prognosis but the outcome depends on the clinical and radiological stage at which the patients present. The earliest stage of this condition does not necessarily progress in every patient, but discriminatory radiological information is lacking in order to differentiate these patients so that unnecessary surgical intervention can be avoided. We describe 20 sequential cases of early SONK diagnosed by magnetic resonance imaging (MRI) in which non-operative management led to the spontaneous resolution of symptoms and MRI changes. Our data was compared to published series in order to derive more accurate prognostic criteria that may then be used to determine appropriate management. Average age of patients was 52 years (42-64). All patients' symptoms resolved with conservative treatment at an average of 4.8 (3-8) months after symptoms began. All MRIs returned to normal. MRI prognostic criteria that appear to indicate a benign course are the absence of focal epiphyseal contour depression and the absence of lines of low signal intensity deep in the condyles. The presence of high signal T2 rim and a length >14 mm and depth >4 mm of the low signal T2 lesion did not necessarily indicate a bad prognosis. Early SONK in this group of patients resolves without surgical intervention. The group is typically middle aged, present with acute focal pain in the knee, have no secondary cause of osteonecrosis, have minimal or no changes on plain radiographs, and have focal changes on MRI. Recognition of this group using MRI identifies the earliest changes in SONK and gives prognostic information that avoids inappropriate surgical intervention.