A given cancer is a disease which combines a paraneoplastic syndrome with an invasive tumour capable of giving rise to metastases. Surgeons, radiotherapists, medical oncologists and experimental scientists are primarily interested in the tumour. Tumours of tissues and organs which do not normally produce hormones might, during the neoplastic transformation, begin to secrete hormones or substances able to mimic hormones in their effects on other tissues in the organism. The number of known hormones has increased considerably in the last 20 years. It has been found that even in the absence of clinical signs there are often secretory abnormalities and changes in the hormone balance in cancer. The tumour-paraneoplastic syndrome interaction is bidirectional. That paraneoplastic syndromes are dependent upon the tumour, is universally accepted; the reverse, that the tumour might depend on the paraneoplastic syndrome is not part of the current way of thinking. To treat cancer patients, instead of debating the cause and effect in the tumour-paraneoplastic syndrome pair with the classical idea of acting as close to the cause as possible, it seems better, in all circumstances, to treat both the tumour and the paraneoplastic syndrome, even if only subclinical.