Since the development of the radioimmunoassay, the determination of serum or plasma levels of peptides has improved the diagnostic possibilities in neuroendocrine gastrointestinal and pancreatic tumors. Once the diagnosis of a carcinoid or an endocrine pancreatic tumor has been established the analyses of various peptides can help to characterize the tumor. Up till now there has been no 'screening' marker for the early detection of these tumors. The development of radioimmunoassay for analyses of chromogranins in plasma might imply such possibilities. In the management of the patients it is also important to have tumor markers that can give prognostic information. Recent findings might indicate that CA-50 could be an indicator of malignant disease and possibly also correlate with survival.