Introduction. Cancer metastases from an unknown primary origin (CUP) in the head and neck area include a large heterogeneous group of tumor formations, the primary localization of which could not be established even after a thorough diagnostic search. Combined positron emission computed tomography (PET-CT) with 18F-FDG has a high level of detection of primary neoplasms as a result of simultaneous acquisition of precisely combined anatomical and functional images of the entire body.Objectives. 1. To evaluate the possibility of using PET-CT (positron emission tomography in combination with computed tomography) with 18F-FDG (18-fluorodeoxyglucose) in patients with metastases from the CUP (carcinoma unknown primary) in the head and neck.2. To determine the effectiveness of PET-CT for the detection of primary tumor focus in patients with histologically verified diagnosis of CUP in the head and neck.3. To evaluate the contribution of the PET-CT technique in assessing the prevalence of the disease in patients diagnosed with head and neck cancer.Materials and methods. 134 patients diagnosed with CUP in the head and neck from September 2018 to March 2019 were included to this retrospective study: 35(26.1%) women and 99(73.9%) men, with the mean age of 61.9 ± 7.5 years. All patients underwent a biopsy from at least one metastatic lesion, and the malignant nature of the neoplasm was histologically verified. Before PET-CT with 18F-FDG, standard oncological evaluation was carried out to detect primary tumor.Results. Among 134 patients comprised the study, initial histological diagnoses were: squamous cell cancer (n = 82, 61.2%), melanoma (n = 5, 3.7%), undifferentiated carcinoma (n = 20, 15%), adenocarcinoma (n = 21, 15.6%) and undifferentiated malignant neoplasm (n = 6, 4.5%).With PET-CT with 18F-FDG, primary tumor location was revealed in 72 (54%) of the patients. New metastatic lesions were found in 60 (44.7%) of the patients.Change in the TNM stage was observed in 95(71%) of all cases after PET-CT with 18F-FDG. It was associated both with the detection of the primary tumor and the identification of new metastatic foci.Conclusions. Use of PET-CT with 18F-FDG allows to carry out oncological evaluation more precisely than the standard procedures in the majority of patients with CUP in the head and neck region. In a significant quota of cases, this method is in a position also to help to identify the primary tumor lesion, which, in turn, influences on strategy of treatment and also on the prognosis of disease. PET-CT with 18F-FDG should be included to the protocol of radiological examination in patients with CUP in the head and neck anatomic area.