Malignant tumours of nonendocrine tissues may produce ectopic hormones. The most likely mechanism is depression of genes which code for hormones. Ectopic hormones are invariably peptides, and each is identical to some peptide product of an endocrine gland. However, the majority of ectopic hormones occur as biologically inactive precursors or subunits and therefore remain occult unless they are specifically sought. When appropriate assays are made for such inactive forms, it is found that ectopic production of hormone-like peptides occurs frequently. Clinical syndromes result only in the relatively rare patients in whom a biologically active form is synthesized in large quantities. Laboratory research in this area improves our understanding of genetic control mechanisms in neoplasia. Ectopic hormones may be of limited use in diagnosis of cancer, especially when multiple markers are measured simultaneously.