Background: There are numerous methods for quantifying the extent of osteonecrosis of the femoral head. However, there is no consensus regarding which method is the most reliable. The purpose of this study was to determine the reliability and prognostic accuracy of three commonly used methods for quantifying the extent of osteonecrosis of the femoral head. Methods: Thirty-nine hips in twenty-five patients who had stage-I or II osteonecrosis of the femoral head, according to the grading system of the Association Research Circulation Osseous, were independently examined on two separate occasions by three observers of different specialty backgrounds and experience. Each observer used three methods to quantify the extent of osteonecrosis of the femoral head: (1) the index of necrotic extent, (2) the modified index of necrotic extent, and (3) the percentage of femoral head involvement. The interobserver and intraobserver agreement was determined for each method, and the ability of each method to predict the time to subchondral collapse was analyzed statistically. Results: There was significantly valid agreement among the observers for all three methods (p < 0.001 for all three). The correlation coefficients demonstrated substantial agreement among raters when they measured the index of necrotic extent and the percent involvement and nearly perfect agreement when they measured the modified index of necrotic extent. Survivorship analysis revealed that the percent involvement (p < 0.05), index of necrotic extent (p < 0.007), and modified index of necrotic extent (p < 0.04) were prognostically significant predictors of subchondral fracture. Conclusions: Our results indicate that the index of necrotic extent, modified index of necrotic extent, and estimation of the percentage of involvement of the femoral head are reproducible and reliable methods for quantitatively evaluating the extent of osteonecrosis of the femoral head. We believe that the three methods can be utilized with confidence. Furthermore, they are clinically useful for identifying hips at greatest risk for subchondral collapse.