Neuropeptide Y and neuron-specific enolase levels in benign and malignant pheochromocytomas. Neuron-specific enolase (NSE) is the isoform of enolase, a glycolytic enzyme found in the neuroendocrine system. Neuropeptide Y (NPY) is a peptide recently discovered in the peripheral and central nervous systems. Serum NSE and plasma NPY levels have been reported to be increased in some patients with pheochromocytoma. The authors evaluated whether the measurement of these molecules could help to discriminate between benign and malignant forms of pheochromocytoma. The NSE levels were normal in all patients with benign pheochromocytoma (n = 13) and elevated in one half of those with malignant pheochromocytoma (n = 13). Plasma NPY levels were on the average significantly higher in the malignant (177.1 +/- 38.9 pmol/l, n = 16) than in the benign forms of the disease (15.7 +/- 389 pmol/l, n = 24). However, there was no difference in the percentage of patients with elevated NPY levels. These results show that determination of serum NSE may be useful for distinguishing between malignant and benign pheochromocytoma; the measurement of plasma NPY is not useful for differentiating the two kinds of tumors. 