Introduction. In osteonecrosis of the femoral head (ONFH), reduction in the size, or complete resolution of the necrotic lesion has been reported to occur spontaneously without any specific treatment. Recently, there was a report that the reduction was time-dependent. We evaluated the change in the size of necrotic lesions of ONFH using magnetic resonance imaging (MRI) more than 10 years after the initial diagnosis. Methods. Fifteen hips in 13 patients with atraumatic ONFH who had been followed-up for more than 10 years were enrolled in this study. They were categorized into two groups; A Simple Observation Group and a Multiple Drilling Group. The Simple Observation Group included 6 hips in 5 patients treated non-operatively. There were 3 men and 2 women who had an average age of 42 years at the time of their initial diagnosis. Initial Ficat and Arlet stages were I in 2 cases, IIA in 3 cases, and IIB in 1 case. The Multiple Drilling Group included 9 hips in 9 patients treated surgically with multiple drilling. They were all men who had an average age of 38 years at the time of operation. There were 2 cases of stage I and 7 cases of stage IIA. The necrotic lesion size change was evaluated by comparing the last follow-up MRI images with the initial images. All of the coronal, sagittal, and axial plane images were reviewed by 2 orthopaedic surgeons and a radiologist. The lesion size change was determined by means of consensus of the reviewers. The lesion size change was defined when it was detected in more than 2 planes. Results. The average time interval between the initial and last MRI imaging was 11.5 years (range, 10 to 16 years). Two cases, 1 case from each group, were excluded from the final evaluation because demarcation of the outer margin of lesion was impossible on MRI images due to severe secondary arthritic changes in the femoral head. Among the 5 cases of the Simple Observation Group, 2 cases showed a decrease in lesion size. In the Multiple Drilling Group, 5 out of 8 cases showed a decrease in lesion size and the lesion disappeared almost completely in 2 of these cases. There was no case of an increase in lesion size in both groups. Conclusion. Reduction in the lesion size of ONFH was found in some cases of long-term follow-up on MRI images. A larger portion of the cases demonstrated lesion size decrease in the Multiple Drilling Group than in the Simple Observation Group