Abstract Objective. The correlation between plasma Chromogranin A concentrations and changes in tumor size evaluated by computed tomography (CT) - as a gold standard - was evaluated. Material and methods. One hundred and sixteen patients with CgA-producing ileo-cecal neuroendocrine tumors were evaluated by events, which were recorded when a CT was followed by another CT 1 - 12 months later. Change in tumor size was defined as regression, progression, or stable disease using RECIST criteria 1.1. Of 426 events, there were 97 with progression, 279 with stable disease, and 50 with regression. Based on the ROC curves a cutoff value of 25% change was selected to discriminate between increased, decreased, or unchanged CgA concentrations in plasma, using a sensitive radioimmunoassay with well-defined epitope specificity. Results. In the 97 events showing tumor progression diagnostic sensitivity and specificity of an increased CgA concentration were 86% and 86%, respectively. The positive and negative predictive values were 64% and 85%, respectively. In the 279 events with unchanged tumor size the diagnostic sensitivity and specificity of an unchanged CgA concentration were 73% and 86%, and the positive and negative predictive values were 91% and 63%, respectively. In the 50 events showing tumor regression, diagnostic sensitivity and specificity of a decrease in CgA concentration were 78% and 91%, the positive and negative predictive values being 55% and 97%. Conclusions. CgA concentrations in plasma have a high diagnostic accuracy in monitoring patients with ileo-cecal neuroendocrine tumors. In particular, an increase in plasma CgA concentration was useful to indicate tumor progression.