OBJECTIVE In this study, we aimed to evaluate the contribution of single-photon emission computerized tomography/computed tomography (SPECT/CT) to three-phase planar bone scintigraphy/SPECT in the assessment of aseptic and septic prosthesis loosening in patients with painful hip and knee prostheses. METHODS Fifty patients who had undergone arthroplasties (20 hips and 30 knees) and were suspected to have complications and had undergone revision surgery were included in this study. Technetium-99m methylene diphosphonate three-phase bone scintigraphy and SPECT/CT were performed at the region of prostheses in all patients. Planar bone/SPECT and SPECT/CT images were separately assessed by two nuclear medicine physicians. SPECT/CT findings were compared with the findings of planar images/SPECT. Both planar bone scan/SPECT and SPECT/CT findings were divided into three groups: aseptic loosening, septic loosening, and miscellaneous. In all patients, scintigraphic diagnosis was confirmed by surgical findings. RESULTS SPECT/CT changed the diagnosis and treatment plan in 8/50 (16%) patients. SPECT/CT was significantly better than planar scan/SPECT imaging for the diagnosis of aseptic and septic loosening in both joints (κ value: 0.477 for planar scan/SPECT; κ value: 0.717 for SPECT/CT). Moreover, both planar scan/SPECT and SPECT/CT were statistically successful in knee prostheses than in hip prostheses (κ value: 0.271 vs. 0.579 for planar/SPECT; κ value: 0.579 vs. 0.80 for SPECT/CT). For the hip, SPECT/CT was successful on the acetabular component than on the femoral component. For the knee, the results of SPECT/CT were similar for the femoral and tibial components. CONCLUSIONS SPECT/CT increases diagnostic accuracy in the evaluation of aseptic and septic loosening in hip and knee prostheses compared with three-phase bone scintigraphy/SPECT.