Though it may be equivocal in the obese patients, the measurement of the retrogastric space in still helpful in the early detection of retroperitoneal masses, such as carcinoma of the pancreas especially in the body and tail of the pancreas. Recently the best method to detect a retroperitoneal mass is, of course, Computed Tomography. But its application to every patient complaining epigastric distress is almost impossible because of its high cost. In those cases the measurement of the retrogastic space is advisable on the erect left lateral film checked during ordinary upper G-1 series as a screening test. New method to measure the retrogastric space is proposed, and its relationship to the body thickness is studied. The results are as follows ; 1. The total number of the patient was 120 and sex distribution was 56 males and 64 females. 2. The retrogastric space and the retropyloric space had a close relationship with the thickness of the body. 3. The average ratio of the retr gastric space to the body thickness was 48.9$pm$8.4% in male and 43.3$pm$10.1% in female. 4. The average ratio of the retropyloric space to the body thickness was 38.6$pm$7.9% in male and 34.9$pm$9.1% in female. 5. The distance of the Treitz ligament from the fundus of the stomach had no relationship with the body thickness.