Being pepsinogen A (PGA) levels generally reduced and pepsinogen C (PGC) increased in gastric cancer patients, PGA/PGC ratio has been proposed as a useful marker of the tumour. We tested PGA, PGC and Gastrin (G) levels in patients with gastric cancer (39) and, as a control, in patients with epithelial dysplasia (21), chronic atrophic gastritis (57), gastric ulcer (11) or subjects lacking major or minor endoscopic and microscopic changes at gastroscopy (48). PGA and PGA/PGC levels were significantly reduced in gastric cancer patients (p less than 0.005 and p less than 0.0001 respectively with analysis of variance). Gastrin levels were also reduced in the same patients (p less than 0.005). We therefore adopted an index number (PGA x Gastrin) which was also dramatically reduced in gastric cancer (p less than 0.005); using an arbitrarily chosen cut-off, the "marker" showed very high sensitivity (76%), specificity (96%) and overall accuracy (74%, by Youden J test). We therefore suggest the use of the index number PGA x G in the diagnosis of gastric cancer, as the most useful gastrin presently available, to our knowledge.