: The aim of this article is provide a survey of the current knowledge relating to the analysis of melatonin and its administration to cancer patients. On the basis of this compilation of data it can be discussed under which conditions melatonin may be used for diagnostic and/or therapeutic purposes in clinical oncology. Melatonin is depressed in patients with cancers of different origins during the phase of primary tumour growth whereas a normal or sometimes elevated pineal melatonin secretory activity is found during early stages of tumour development or when recidivations arise. The clinical studies of Lissoni show that melatonin, particularly if combined with interleukin-2, is able to favourably influence the course of advanced malignant disease leading to a prolonged survival as well as to an improved quality of life. These findings require to be verified by independent and controlled replication studies. If they can be confirmed it should be attempted to administer melatonin to patients with earlier stages of cancer parallel to standard oncological treatment regimens. In such trials it should be tested whether a substitutional therapy in patients with endogenously depressed melatonin may favourably affect the course of the disease both in quantitative (inhibitory effect on tumour growth and spread) and qualitative terms (improved performance status).