The naturally occurring polyamines, spermine, spermidine and the diamine putrescine are widespread in nature. They have been implicated in growth and differentiation processes. Polyamines accumulate in cancerous tissues and their concentration is elevated in body fluids of cancer patients. Assays of urinary and blood polyamines have been used to detect cancer and to determine the success of therapy. Drugs which inhibit the synthesis of polyamines can prevent cancer and may also be used for therapeutic purposes. Ornithine decarboxylase, which catalyzes the rate limiting step in polyamine synthesis, can serve as a marker of proliferation. Recently, a new in vitro chemosensitivity test, based on the disappearance of ornithine decarboxylase in drug-treated cancer cells has been developed. The increasing interest in polyamines and their physiological functions may lead to a more extensive application of these compounds or their derivatives in cancer diagnosis and treatment.