Positron-emission tomography (PET) imaging has several advantages over conventional scintigraphy, including a high spatial resolution and the ability to quantify disease progression. Recently, (18)F-fluoride PET has been applied to the evaluation of malignant tumors and musculoskeletal disorders. In our current study, spontaneous osteonecrosis of the knee (SONK) was visualized using this technique. We determined whether PET images can reveal SONK lesions, whether there were significant differences in the maximum standardized uptake value (SUVmax) among each of the SONK stages, and finally if there was any correlation between the maximum SUVmax value and size of the SONK lesion measured both by radiography and MRI. Fourteen knees from 13 patients diagnosed with SONK were imaged using a PET scanner. In all cases, PET showed an accumulation of 18F-fluoride in the medial condyle. The SUVmax ranged from 8.6 to 23.7 with an average of 15.1 +/- 3.7 and was measured at different disease stages with an average of 12.4 +/- 5.9 in Stage 2 (n = 5), 16.3 +/- 1.4 in Stage 3 (n = 4), and 16.8 +/- 4.3 (n = 5) in Stage 4 lesions. There were no significant differences in these measurements between the SONK stages. However, a significant positive correlation between the SUVmax and lesion size, including the surface area of the lesion (r2 = 0.692, P = 0.0002) and the condyle width ratio (r2 = 0.365, P = 0.022), was found. The approximate volumes of the lesions measured by MRI had an average of 4.8 +/- 3.1 cm3, and also showed a significant correlation with the SUVmax (r2 = 0.853, P < 0.0001). Hence, our present results indicate that a high SUV is indicative of a large SONK lesion.