Chromogranin A (CgA), a protein stored in the secretory granules of many neuroendocrine cells and neurons, is believed to play intracellular and extracellular functions as a regulator of secretory granules biogenesis and as a precursor of various regulatory peptides of the endocrine and the metabolic systems. CgA is abnormally expressed by various neuroendocrine tumors and is released in high amounts in the blood stream. Detection of CgA in tumor tissues and in biological fluids has proven useful for tumor diagnosis and for monitoring tumor progression / regression after therapy. However, little is known on the effect of excessive production of CgA on tumor behavior. A growing body of evidence suggests that tumor-derived CgA is not only an important diagnostic and prognostic marker, but that it could also play a role in tumor / host interaction.