Cancer of unknown origin (CUO) is defined by the absence of any primary tumour in biopsy-proved metastatic cancer. CUO accounts for a 5-10% of all malignancies. These tumors have a specific biology with clinical characteristics of rapid progression and atypical metastases. Diagnostic evaluation is directed at the identification of treatable subset. Accurate diagnostic workup is crucial because both prognosis and survival rates depend mainly on detection of the primary tumor site. Although these patients undergo extensive imaging procedures, nuclear medicine techniques are under-utilized despite their ability of providing molecular information. Positron emission tomography has an emerging role in this clinical challenge along with other nuclear medicine methods including, bone scan, thyroid scintigraphy.