Serum levels of pancreatic polypeptide (PP) were determined in 32 normal subjects, 18 patients with gastrinoma, 19 patients with pituitary adenoma and 11 patients with duodenal ulceration. Mean serum levels of PP in patients with gastrinomas (67 +/- 31 pmol/l (SD)) were significantly (P less than 0.01) higher than in the normal subjects (47 +/- 15 pmol/l). However, only five of 18 of the patients with gastrinomas had PP levels above the upper limit of normal and elevations of similar magnitude were observed in patients with other disease processes. It is therefore concluded that serum PP estimations are not of value in the diagnosis of gastrinomas.