Since GIST hardly produce any symptoms, they are often quite large before they are discovered. Those tumors are often discovered during emergency surgery for unexpected perforation of the gastrointestinal tract and consequent bleeding.
The type of symptoms and when they first occur depends on the size and location of the tumor.

If the tumor is situated in the stomach or the duodenum first signs could be a feeling of fullness, pain, gastrointestinal hemorrhages (black stool) or nausea. Tumors in the small intestine often reach a noticeable size before the pressure they cause on other organs leads to pain, haemorrhages, or constipation. Tumors in the large intestine can cause bloody stool and constipation. Primary tumors in the esophagus can lead to trouble swallowing.