A review of findings is given which relate to the levels of circulating melatonin as well as the urinary excretion of its main peripheral metabolite 6-sulphatoxymelatonin (aMT6s) in patients with different types of cancer as well as in tumor-bearing animals. Clinical results show that circulating melatonin tends to be depressed in patients with primary tumors of different histological types including both endocrine-dependent (mammary, endometrial, prostate cancer) and endocrine-independent tumors (lung, gastric, colorectal cancer). Reduction of melatonin is most pronounced in patients with advanced localized primary tumors, such as mammary and prostate cancer where a clear negative correlation with tumor-size exists. The phenomenon of a reduction of circulating melatonin appears to be a transient one since patients with recidives show a normalization of melatonin. Surgical removal of the primary tumor does, however, not lead to normalization indicating that complex systemic changes appear to be involved in the down-regulation of melatonin. It is unclear at present, whether circulating melatonin is depleted in cancer patients due to a reduced production by the pineal gland or due to certain peripheral metabolic processes, although no evidence for an enhanced hepatic degradation to aMT6s, the main peripheral metabolite of melatonin, was found. The reduction of circulating melatonin is accompanied by neuroendocrine changes affecting the circadian secretion of the adenohypophyseal hormones prolactin, somatotropin and thyroid-stimulating hormone. In contrast to the above-described types of tumors many patients with ovarian cancer show highly elevated levels of melatonin perhaps due to the production of tissue-specific growth factors that could affect pineal melatonin secretion. Experiments with tumor-bearing animals clearly demonstrate that nocturnal circulating melatonin is modulated due to malignant growth. Detailed investigations with chemically induced mammary tumors in rats and serial transplants derived thereof show that slow-growing and well-differentiated tumors containing epithelial cell elements (adenocarcinomas and carcinosarcomas) lead to an enhanced production of melatonin involving activation of the rate-limiting enzyme of pineal melatonin biosynthesis (serotonin N-acetyltransferase) probably due to elevation of the sympathetic tone in response to a stimulation of the cellular immune system by malignant growth. As opposed to that nocturnal melatonin is depleted in animals with fast-growing mammary tumor transplants when myoepithelial-mesenchymal conversion leads to pure sarcomas. The reduction of melatonin appears to be due to either a reduced availability of the precursor amino acid tryptophan because of a glucocorticoid-induced activation of the hepatic enzyme tryptophan 2,3-dioxygenase or a direct peripheral degradation of melatonin via indoleamine 2,3-dioxygenase expressed in tumor and/or other tissues. The significance of these clinical and experimental findings relating to melatonin is discussed both in terms of their practical application as a possible tumor marker and from a theoretical point of view to understand better the mechanisms involved in complex host-tumor interactions involving the neuroimmunoendocrine network.