Forty-eight total hip arthroplasties for which revision surgery was performed were reviewed to determine the accuracy of laboratory tests, plain radiographs, hip aspiration, and technetium-99m MDP and gallium-67 scans in demonstrating the presence or absence of infection of the prosthesis. Six of the 48 hips were diagnosed as having an infection at the revision surgery. The erythrocyte sedimentation rate and the C-reactive protein levels were significantly higher in the patients with infected prostheses. The difference in the white blood cell count was not significant. There was no significant relationship between the presence of infection and the severity of loosening and instability of the implants diagnosed by plain radiographs. The accuracy of hip aspiration in diagnosing the infection was 83%, with a sensitivity of 40% and a specificity of 92%. The accuracy of technetium-99m MDP bone scan was 79%, with a sensitivity of 83%, and a specificity of 79%. Gallium-67 scan had an accuracy of 96%, a sensitivity of 67%, and a specificity of 100%. The findings in the present study indicated that diagnostic tests consisting of laboratory tests and plain radiography, followed by hip aspiration and sequential use of technetium-99m MDP and gallium-67 scintigraphies, are suitable for differentiation between mechanical loosening and infection of total hip arthroplasty.