In most cases, GISTs diagnosis is an incidental finding in instrumental exams performed to investigate other diseases.
The most indicative exams are:
Endoscopy and ultrasonography: extremely important for oesophageal, gastric, duodenal and anorectal lesions.

Endoscopic ultrasonography: a new technique which consists of an ultrasonography exam of gastrointestinal tract performed during endoscopy. Especially useful for the detection of submucosal masses of the gut wall and to permit evaluation of the loco-regional infiltration.

Computed tomography (CT): a fundamental exam to asses tumour extension for correct staging and surgical planning. Useful mostly for small bowel and omentum lesions, when endoscopy cannot be performed.

Magnetic resonance imaging (MRI): is the gold standard for surgical planning of GISTs localized in regions not assessable with other methods such as the anorectal one.

PET-CT (computed tomography-positron emission tomograpghy): it asses metabolic activity of the tumour correlating the anatomic with functional features of the tumour. Useful in baseline disease staging, in the treatment planning and in early response evaluation to therapy.

Biopsy: can be performed by non-surgical biopsies using FNA during endoscopy, ultrasonography and CT procedures or by surgical procedures with laparoscopic or laparotomic access. Histological and immunohistochemical analysis are then performed on the samples collected.
However, performing biopsies or not in GISTs patients is still controversial.