a disease characterized by abnormal growth of cells in the stomach. The incidence of stomach cancer has decreased dramatically since the early 20th century in countries where refrigeration has replaced other methods of food preservation such as salting, smoking, and pickling. Stomach cancer rates remain high in countries where these processes are still used extensively.

Ninety-five percent of malignant stomach cancers develop from epithelial (epithelium) cells lining the stomach. These tumours are called adenocarcinomas. Other stomach cancers can develop from the surrounding immune cells, hormone-producing cells, or connective tissue. Multiple risk factors have been identified that increase a person's probability of developing this cancer. These include a diet high in salted, smoked, or pickled foods, tobacco and alcohol use, or a family history of stomach cancer. Infection by the bacterium Helicobacter pylori, which can cause significant damage to gastric tissues and is a cause of peptic ulcers (peptic ulcer), can also lead to stomach cancer. Other factors that may increase the risk of stomach cancer to varying degrees are previous stomach surgery, blood type A, advanced age (60–70 years), or chronic stomach inflammation. Males develop stomach cancer at approximately twice the rate of females. Rare disorders such as pernicious anemia or Menetrier disease and congenital disorders that lead to increased risk for colorectal cancer may also increase stomach cancer risk.

The symptoms of stomach cancer are prevalent in many other illnesses and may include abdominal pain or discomfort, unexplained weight loss, vomiting, poor digestion, or visible swelling in the abdomen.

No specific laboratory test for stomach cancer exists, and the disease is therefore usually diagnosed through a combination of visual means. A physician can inspect the lining of the stomach with a flexible, lens-containing tube called an endoscope (endoscopy). The endoscope can also be used to take samples from potentially cancerous tissues for biopsy. These samples are examined under a microscope for signs of cancer. An endoscope may also be modified with a special probe that emits sound waves in the stomach, which allows the physician to create an image (diagnostic imaging) of the stomach wall. X rays are also employed, usually after the patient has swallowed a barium compound that coats the stomach and provides better image contrast. Other imaging techniques such as computed tomography (computerized axial tomography) (CT) scans and magnetic resonance imaging (diagnosis) (MRI) are also used, especially when the cancer is believed to have spread.

Once stomach cancer has been diagnosed, its stage is determined. The stage is an indicator of how far the cancer has progressed. Staging for stomach cancer is complicated and is based on a combination of how far the cancer has grown through the stomach wall and on the number of lymph nodes affected, if any. Stage 0 stomach cancer is also called carcinoma in situ and is confined to the epithelial cells that line the stomach. Stage I and stage II cancers have spread into the connective tissue or muscle layers that underlie the epithelial cells, but they have reached fewer than six nearby lymph nodes. Stage III and IV cancers are more advanced and may have metastasized to distant tissues.

A very high percentage of individuals survive stomach cancer for at least five years if the cancer is diagnosed very early, and many of them go on to live long, healthy lives. Unfortunately, only a small percentage of stomach cancers are identified and treated at such an early stage. At the time when most lower-stomach cancers are diagnosed, roughly half the patients survive for at least five years. Cancers of the upper stomach have a lower survival rate, and if the cancer has spread to distant tissues in the body, the survival rate is extremely low.

Stomach cancer cannot be completely prevented, but people can decrease their risk of disease by adopting a diet that is low in salted, smoked, and pickled foods and high in fruits and vegetables. Elimination of tobacco use and reduction in alcohol consumption also help lower risk. Research has indicated that prompt treatment of H. pylori infection can reverse gastric tissue damage, thereby reducing stomach cancer risk.