The term cancer of unknown primary site includes metastatic tumours with different histology and behaviour. Although most of them have a poor short-term prognosis, some patients can benefit from a treatment and will achieve a longer survival. The treatable cases are: metastases of squamous carcinoma in cervical or inguinal adenopathies, metastases of adenocarcinoma in axilar adenopathies in women, malignant ascites due to adenocarcinoma in women, osteoblastic bone metastases in men with elevated serum prostatic specific antigen levels, poorly differentiated tumours with features of a germinal extragonadal tumour, poorly differentiated neuroendocrine carcinomas and patients with a single metastasis. Chemotherapy must be considered in the rest of patients, although the optimum regimen is not well established yet.