Stomach cancer is the result of cell changes in the lining of the stomach. In Western Europe, stomach cancer is only about one third as common as it was 50 years ago.

The location of the tumours within the stomach has also changed. It used to be that most of the tumours were located near where the stomach joins the small bowel, now the commonest site is close to the junction with the oesophagus (gullet).

Stomach cancer is very common in Japan and the Japanese have developed an intensive and effective approach to screening for stomach cancer.

Stomach cancer can grow slowly and imperceptibly. Sometimes symptoms will only develop once the disease has spread beyond the stomach, for example to involve the liver. In this case, the symptoms would be those of cancer involving the liver. There may be internal bleeding appearing as blood in the vomit, or black, tar-like, faeces, or the bleeding may be so slight as to pass undetected, and the patient goes to the doctor with iron-deficiency anaemia. Or there may be vomiting due to the obstruction of the food passage by the tumour, which is the first sign that something is wrong. Excessive belching can be an early, and embarrassing, symptom. This can progress to a sensation of vague discomfort followed by pain if the tumour grows through the stomach wall.

As with cancer of the oesophagus, it may take a long time, often many months, from the time that symptoms first appear to the patient seeking medical advice. This delay may allow time for the tumour to spread and to progress from being potentially curable to being inoperable.