Neuron-specific enolase (NSE), the glycolytic isoenzyme of the enolase gamma-gamma dimer, is a specific marker for the diffuse neuroendocrine system and derivative tumors (NET). Serum levels of NSE were measured in 39 patients with NET of the gastrointestinal tract (including 3 gastric and 13 intestinal carcinoid tumors, 6 gastrinomas, 3 insulinomas, 1 gluca-gonoma, 2 mixed islet cell tumors, 11 neuroendocrine pancreatic carcinomas), in 15 healthy subjects and in 15 nonendo-crine gastric, pancreatic, and intestinal tumors. Thirty-six of the 39 patients had elevated circulating levels of NSE, 2 insulinomas and 1 gastrinoma had values below 12 ng/ml like healthy subjects and nonendocrine tumors. No significant difference of serum NSE was found between 23 ‘functioning’ and 16 ‘nonfunctioning’ NET. Fourteen of the NET were malignant, and NSE circulating values were significantly higher than those of nonmalignant forms. After curative surgery serum NSE decreased significantly. NSE can be considered a reliable marker in the differential diagnosis between endocrine and nonendocrine neoplasms, in the clinical detection of silent endocrine tumors and in the follow-up of NET.