It is difficult to predict if and when the femoral head will collapse and whether the collapse can be minimized. This study examined the final outcome of early-stage osteonecrosis of the femoral head (ONFH) using magnetic resonance imaging to verify the relationship between preservation of the lateral pillar and collapse of the femoral head. A midcoronal section of the femoral head was divided into 3 pillars (medial, central, and lateral) on a T1-weighted image. According to the site of necrosis on the lateral pillar, the necrosis was divided into 3 types: I, the necrosis occupies the central and medial pillars, and the lateral pillar is preserved; II, the necrosis partially occupies the lateral pillar; and III, the necrosis totally occupies the lateral pillar. One group of 87 patients (127 hips) with Association for Research on Osseous Circulation (ARCO) stage I ONFH underwent nonoperative treatment and were followed for 3 to 8 years (average, 6.2 years). Another group of 42 patients (72 hips) with ARCO stage I ONFH underwent debridement and impacted bone graft and were followed for 5 to 9 years (average, 7.1 years). In both groups, the more preserved the lateral pillar, the less collapse occurred. The authors concluded that whether ONFH progressed to collapse is determined by preservation of the lateral pillar. The lateral pillar is the keystone for maintaining the sphere of the femoral head and its preservation.