The term gastrointestinal peptide is usually used to refer to a group of peptides with relatively low molecular weight (generally <30 amino acids) that function as agonists by interacting with cell surface receptors on one or more gastrointestinal target cells. These peptides can function as hormones or neurotransmitters, or can act as paracrine regulators of target cell function. Recently it has become apparent that some gastrointestinal peptides may play important roles in influencing the rates of proliferation of certain neoplastic cells. In the present article, we highlight major areas of research that provide information about the relationships between gastrointestinal peptides and cancer. In addition to identifying what we believe is a "trend," we also make suggestions as to likely future developments.