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Several radiographic parameters utilized for the diagnosis of acetabular dysplasia in adults suffer from poor reproducibility and reliability.To define and validate a novel radiographic parameter (the iliofemoral line [IFL]) for the detection of frank and borderline hip dysplasia and to compare the sensitivity and specificity of this radiographic marker to those of previously validated qualitative parameters.Cohort study (diagnosis); Level of evidence, 2.A consecutive cohort of 222 adult patients (436 hips) undergoing hip preservation surgery was included. The IFL, which extends from the lateral femoral neck through the inner cortical lip of the iliac crest, intersects the femoral head in cases of dysplasia. Percent medialization of the IFL was defined as the horizontal distance of the exposed femoral head lateral to the IFL, relative to the horizontal femoral head width at the center of the femoral head.Percent medialization of the IFL was strongly correlated to the lateral center edge angle (P < .0001). Values of percent medialization ranging from 15% to 22% predicted the presence of borderline hip dysplasia with a sensitivity of 62% and specificity of 89%, while values exceeding 22% predicted the presence of frank acetabular dysplasia with a sensitivity of 77% and specificity of 94%. By comparison, abnormality of the Shenton line demonstrated a sensitivity of 3.7% and specificity of 97% for the detection of borderline dysplasia and a sensitivity of 16% and specificity of 99% for the detection of frank acetabular dysplasia. Compared with the Shenton line, percent medialization of the IFL was significantly more sensitive for the detection of both borderline and frank acetabular dysplasia (both P < .0001). The intraobserver and interobserver reproducibility of the horizontal difference outside the IFL were 0.99 and 0.96, respectively.Percent medialization of the IFL is a reliable and accurate radiographic marker of frank acetabular dysplasia and, to a lesser extent, borderline dysplasia. The use of this radiographic parameter as an additional tool may enable the earlier detection of borderline and frank hip dysplasia in young adults presenting with hip pain.