The aim of this study was to assess the usefulness of PET with 2-¹⁸F- fluoro-2-deoxy-D-glucose (FDG), as compared to immunoscintigraphy (IS) with ^99mTc-labeled monoclonal antigranulocyte antibodies (AGAbs), in the detection of chronic osteomyelitis. Methods: Fifty-one patients suspected of having chronic osteomyelitis in the peripheral (n = 36) or central (n = 15) skeleton were evaluated prospectively with static FDG PET imaging and combined ^99mTc-AGAb/^99mTc-methylene diphosphonate (MDP) bone scanning within 5 days. FDG PET and IS were evaluated in a blinded and independent manner by visual interpretation, which was graded on a five-point scale of two observers' confident diagnosis of osteomyelitis. Receiver operating characteristic (ROC) curve analysis was performed for both imaging modalities. The final diagnosis was established by means of bacteriologic culture of surgical specimens and histopathologic analysis (n = 31) or by biopsy and clinical follow-up over 2 yr (n = 20). Results: Of 51 patients, 28 had osteomyelitis and 23 did not. According to the unanimous evaluation of both readers, FDG PET correctly identified 27 of the 28 positives and 22 of the 23 negatives (IS identified 15 of 28 positives and 17 of 23 negatives, respectively). The area under the ROC curve was 0.97/0.97 (reader 1/reader 2) for FDG PET and 0.87/0.90 for IS, with a high degree of interobserver concordance (κ-values were 0.96 for FDG PET and 0.91 for IS). In the central skeleton, the ROC curve area was 0.98/1.00 for FDG PET and 0.71/0.77 for IS (p < 0.05). On the basis of ROC analysis, the overall accuracies of FDG PET and IS in the detection of chronic osteomyelitis were 96%/96% and 82%/88%, respectively. With regard to the optimal threshold values, sensitivity and specificity were 100%/97% and 95%/95% with FDG PET, compared to 86%/92% and 77%/82% with IS, respectively. Conclusion: In the peripheral skeleton, both FDG PET and combined ^99mTc-AGAb/^99mTC-MDP scanning are appropriate imaging modalities to diagnose chronic osteomyelitis. FDG PET additionally allows reliable differentiation between osteomyelitis and infection of the surrounding soft tissue. In the central skeleton within active bone marrow, FDG PET is highly accurate and superior to AGAb imaging in the diagnosis of chronic osteomyelitis, which frequently presents as a nonspecific photopenic lesion at scintigraphy with labeled white blood cells.