Objective To investigate the diagnostic and prognostic values of 18 F?FDG PET/CT in pa?tients with cervical nodal metastases from carcinoma of unknown primary site (CCUP). Methods A total of 137 consecutive patients(95 males, 42 females, age range 24-84 (55.77±11.71) years) with histological proven CCUP who underwent 18 F?FDG PET/CT imaging to find primary tumors from March 2010 to June 2015 were retrospectively analyzed. The gold standard was the final pathological diagnosis or clinical long?term fol?low?up (≥6 months) results. The diagnostic sensitivity, specificity, accuracy, positive predictive value and negative predictive value of PET/CT imaging in detecting primary tumors were calculated. Kaplan?Meier anal?ysis was conducted for survival analysis and to identify the presence of any prognostic factors, such as age, gender, presence or absence of distant metastasis, region of involved cervical lymph nodes, left (right)?side/both sides of lymph node involvement, histopathologic tumor type and primary site found or not by PET/CT imaging. Results Primary tumors were confirmed in 96 patients, of which 87 patients were diagnosed correct?ly by 18 F?FDG PET/CT imaging. The diagnostic sensitivity, specificity, accuracy, positive predictive value and negative predictive value were 90.6% (87/96), 80.5% (33/41), 87.6% (120/137), 91.6% (87/95) and 78.6% (33/42) respectively. Primary tumors were found most commonly in the head, neck and lung. However, the false positive and negative lesions were also mainly seen in those sites. Distant metastasis and the inferior region of cervical lymph node metastasis were associated with the worst survival (χ2=20. 990 and 12?277, both P<0.01) . The ROC curve revealed that patients with SUVmax>6.5 were of higher mortality risk than those with SUVmax≤6.5 (χ2=7.120, P<0.01) . Conclusions Whole?body 18 F?FDG PET/CT scan is a significant and valuable tool in the identification of the primary tumor site in patients with CCUP . Further? more, according to the distant metastasis and region of cervical lymph nodes metastasis, 18 F?FDG PET/CT scans are of value in prognostic evaluation even if without the need to identify the primary tumors.