This study aimed to examine the association between hip joint effusion volume and osteonecrosis of the femoral head (ONFH) using the Association Research Circulation Osseous (ARCO) classification. Patients (n = 403) who were diagnosed with ONFH were enrolled between February 2005 and December 2008. Only patients (n = 109) with complete clinical and imaging data and at early to mid ARCO stage (I - III) were eligible for further analysis, including 94 males and 15 females. All the included patients had hip joint radiographic examinations (anteroposterior and frog-leg views) and magnetic resonance imaging scans (axial and coronal views). Out of 109 patients included in this study, 185 hip joints were involved (unilateral disease in 33 patients and bilateral diseases in 76 patients). The patients had a mean age of 39 +/- 11 years (range, 13-70). All the affected hip joints exhibited effusion, classified as grade 1 (n = 70, 37.8%), grade 2 (n = 62, 33.5%), and grade 3 (n = 53, 28.7%). The volume of joint effusion varied significantly among stage I, II, and III (X-2 = 29.210, P < 0.05). The effusion volume did not differ significantly among stage IIA, IIB, and IIC (X-2 = 0.103, P > 0.05), whereas it differed significantly among stage IIIA, IIIB, and IIIC (X-2 = 11.556, P < 0.05). The volume of hip joint effusion was associated with the ARCO stage, and increased over the staging.