We compared the efficacies of 18 F-FDG PET and 99m Tc-bone scintigraphy for the detection of bone metastases in patients with differentiated thyroid carcinoma (DTC). Methods: We examined 47 patients (32 women, 15 men; mean age 6 SD, 57.0 6 10.7 y) with DTC who had undergone total thyroidectomy and were hospitalized to be given 131 I therapy. All patients underwent both whole-body 18 F-FDG PET and 99m Tc-bone scintigraphy. The skeletal system was classified into 11 anatomic segments and assessed for the presence of bone metastases. Bone metastases were verified either when positive findings were obtained on .2 imaging modalities-201 Tl scintigraphy, 131 I scintigraphy, and CT-or when MRI findings were positive if vertebral MRI was performed. Results: Bone metastases were confirmed in 59 of 517 (11%) segments in 18 (38%) of the 47 study patients. The sensitivities (visualization rate) for bone metastases on a segment basis using 18 F-FDG PET and 99m Tcbone scintigraphy were 50 of 59 (84.7%) and 46 of 59 (78.0%), respectively; the difference between these values was not statistically significant. There were only 2 (0.4%) false-positive cases in a total of 451 bone segments without bone metastases when examined by 18 F-FDG PET, whereas 39 (8.6%) were falsepositive when examined by 99m Tc-bone scintigraphy. Therefore, the specificities of 18 F-FDG PET and 99m Tc-bone scintigraphy were 449 of 451 (99.6%) and 412 of 451 (91.4%), respectively; the difference between these values was statistically significant (P , 0.001). The overall accuracies of 18 F-FDG PET and 99m Tcbone scintigraphy were 499 of 510 (97.8%) and 458 of 510 (89.8%), respectively; the difference between these was also statistically significant (P , 0.001). Conclusion: The specificity and the overall accuracy of 18 F-FDG PET for the diagnosis of bone metastases in patients with DTC are higher than those of 99m Tc-bone scintigraphy, whereas the difference in the sensitivities of both modalities is not statistically significant. In comparison with 99m Tc-bone scintigraphy, 18 F-FDG PET is superior because of its lower incidence of false-positive results in the detection of bone metastases of DTC. Skel etal imaging by 18 F-FDG PET has been shown to be useful in the detection of bone metastases of breast (1-6), lung (1,4,7,8), thyroid (4), esophageal (4,9), gastric (4), colorectal (4), endemic nasopharyngeal (10), renal cell (11), prostate (1), ovarian (4), and testicular (4) carcinomas. In most of these studies, 18 F-FDG PET was proven to be superior to conventional scintigraphic imaging using 99m Tclabeled phosphate compounds ( 99m Tc-methylene diphosphonate [ 99m Tc-MDP] or 99m Tc-hydroxymethylene diphosphonate [ 99m Tc-HMDP]). For the detection and evaluation of bone metastases of various kinds of carcinomas, 99m Tc-bone scintigraphy has been used widely because of its overall high sensitivity and the easy evaluation of the entire skeleton (12). However, 99m Tc-bone scintigraphy leads often to falsepositive lesions and, consequently, its specificity is r...