Gastrin has been identified as the principal effector of gastric secretion, but several studies have demonstrated its role as a biomarker of cancer risk and as a growth factor for colorectal, stomach, liver, and pancreatic cancer. Hypergastrinemia characterizes autoimmune gastritis, with body and fundic gland atrophy and increased risk for both gastric adenocarcinoma and neuroendocrine tumors. Gastric type I carcinoids develop in the context of autoimmune gastritis because of the stimulus exerted by gastrin on enterochromaffin-like cells and remain gastrin-sensitive for long durations because the removal of hypergastrinemia leads to tumor regression. The treatment of gastric carcinoid is still open to debate, but when the disease frequently relapses, or is multicentric or infiltrating, surgery is advocated or, in the alternative, a costly and long-lasting treatment with long-acting somatostatin analogues is prescribed. A technology allowing the preparation of an immunogen eliciting an immune system response with generation of antibodies against G17 has been developed. This vaccine has been tested in patients with colorectal, pancreatic or advanced gastric cancer. The vaccine has also been used in the treatment of gastric type I carcinoids, and the administration of G17DT in patients harboring these lesions leads to carcinoid regression. Antigastrin vaccination in the treatment of gastrointestinal cancer obviously needs validation, but this immunotherapy may well represent a simple, inexpensive, and active 'adjuvant' treatment.