Evaluation of submucosal lesions of the digestive tract with conventional endoscopy is unsatisfactory since this technique does not allow direct observation or correct evaluation of the size and layer of origin of the tumor; therefore, in most patients an etiological diagnosis cannot be established with this procedure. However, in most patients, endoscopic ultrasonography can resolve these problems: to a fair degree of certainty, this technique can differentiate malignant from benign lesions, measure their size, and establish their layer of origin. Endoscopic ultrasonography is the technique of choice to establish the presence and characteristics of submucosal tumors and their suitability for treatment. Moreover, this procedure can identify tumors that can be removed endoscopically without excessive risk. Hydrogastric ultrasonography can be an effective substitute for echoendoscopy when evaluating submucosal lesions and for staging tumors of the gastric antrum when echoendoscopy is not available or in patients in whom it cannot be performed. Hydrogastric ultrasonography is safe, inexpensive and very well tolerated by patients. We present the case of a female patient with a gastric GIST that was evaluated using hydrogastric ultrasonography. The size, layer of origin, and malignancy of the tumor were accurately established.