Neuron-specific enolase (NSE) (a glycolytic enzyme that occurs only in neuroendocrine cells), human pancreatic polypeptide (hPP) (a probable peptide hormone), and the beta-subunit of human chorionic gonadotropin (beta hCG) (a glycoprotein hormone), have been suggested as serum markers for the diagnosis and management of apudomas of the pancreas and intestinal tract. Serum levels of NSE, hPP, and beta hCG were measured in 21 patients with pancreatic islet cell cancer (including nine "nonfunctioning" tumors, six gastrinomas, three glucagonomas, two insulinomas, and one somatostatinoma) and in five patients with intestinal carcinoid tumors. Sixteen of the 26 patients (62%) had elevated circulating levels of at least one tumor marker. All three markers were elevated in only one patient, a woman with a "nonfunctioning" islet cell carcinoma. Five patients had increased levels of two serum markers. In three patients both NSE and hPP levels were above normal and in two patients both NSE and beta hCG levels were above normal. In nine patients only one marker was elevated. These included six with elevated NSE levels, two with elevated hPP levels, and one with elevated beta hCG levels. Thus NSE levels were elevated in 12, hPP levels in six, and beta hCG levels in four of the 26 patients. While no single marker was elevated in all patients, each of the three can be elevated in an individual case. When elevated, these markers may be valuable to estimate prognosis or monitor response to therapy.