Background: Osteonecrosis of the femoral head has a host of etiologies. However, in 30% of the cases, no underlying etiology is identified and the process is considered ``idiopathic.'' Our hypothesis was that osseous anatomical abnormalities might be frequently found in patients with femoral head osteonecrosis. Methods: A retrospective, 1: 2 matched, case-control study was undertaken to compare ninety patients with idiopathic osteonecrosis who had undergone lower-limb computed tomography (CT) prior to undergoing total hip arthroplasty with 180 control patients matched for age, sex, and body mass index (BMI) who had undergone lower-limb computed tomography scanning at our radiology department for any reason except articular or bone disease. Preoperative CT scans were performed for all patients to evaluate femoral offset, femoral neck-shaft angle, femoral neck version, femoral head diameter, acetabular coverage estimated with use of the lateral center-edge angle, acetabular version, and acetabular diameter. The mean age was forty-five years, 69% of the patients were men, and the mean BMI was 26 kg/m(2). Results: Anatomical ``abnormalities'' associated with osteonecrosis were determined to be a femoral neck-shaft angle of <129 degrees (likelihood ratio [LR] = 3.6), femoral neck version of >17 degrees (LR = 3.8), a lateral center-edge angle of <32 degrees (LR = 5.7), and acetabular version of <19 degrees (LR = 1.38). A combination of three of the four anatomical ``abnormalities'' was found in 73% of the patients with osteonecrosis but only 11% of the control cases (LR = 6). Conclusions: This 1: 2 matched, anatomical study suggests that acetabular and femoral anatomical factors, in isolation or combination, can be found in a large percentage of cases of ``idiopathic'' osteonecrosis.