Objective: the aim of this study was to evaluate the role of FDG-PET-CT in detection of the primary tumor in patients with bone lesions of unknown origin. Methods and materials: The study is designed as prospective case control study .Data will be obtained prospective 50 patient admitted to the oncology department of benha university or presented to its out patients clinic. The study will be conducted in the PET-CT Unit in ( Life Scan Center ) Results: The study included 50 patients with higher male predominance (68%). Most of patients aged above 40 years old (76%) .Most of lesions were osteolytic lesions (80%) while 30% were sclerotic and 16% were mixed. Most of lesions were multifocal (66%) while 32% were focal lesions and 4% were diffuse . Minimum SUV of the bony lesions ranged from 3to 13 with median 5 and maximum SUV ranged from 5 to 35 with median 15. PET scan was positive for 45 lesions and negative for 5 lesions. Out of 45 positive PET lesions, primary lesions outside the bone were detected in 30 patients while 15 lesions were positive to be primary lesions in the bone. Conclusion: PET CT is the modality of choice for optimal detection of primary tumors is cases of osseous bone lesions of unknown primary.