Gastroenteropancreatic neuroendocrine tumors (GEP-NETs) are relatively indolent but can be more aggressive. The current recommendations for the use of serum CgA for GEP-NET patients are equivocal. This study was designed to validate an automated CgA immunofluorescence assay for monitoring disease progression in GEP-NET patients. A prospective, multi-center blinded observational study was designed to validate an automated chromogranin A (CgA) immunofluorescence assay for monitoring disease progression in GEP-NET patients. Tumor progression was evaluated with Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 by CT/MRI. An increase ≥ 50% above the prior CgA concentration to a value > 100 ng/mL in the following CgA concentration was considered positive. 153 GEP-NET patients were enrolled. Using the pre-specified cut-off of CgA change for tumor progression, specificity was 93·4% (95%-CI: 90·4%-95·5%, p < 0·001), sensitivity 34·4% (25·6%-44·3%), positive predictive value 57·9% (45·0-69·8), negative predictive value (NPV) 84·3% (80·5-87·6), and area under the curve 0·73 (0·67-0·79). Changes in serial measurements of serum CgA had a favorable specificity and NPV, making this test a useful adjunct to routine radiographic monitoring.