Elevated circulating levels of chromogranin A (CgA) are found in the neuroendocrine tumors (NETs), but diagnostic usefulness of this marker is still debatable. To assess the role of CgA for the identification and follow up of gastroenteropancreatic neuroendocrine tumors (GEP-NET), a multicenter prospective longitudinal study has been carried out in Argentina. CgA was measured by RIA in 119 histologically proven GEP-NET patients and in 39 healthy controls. A cutoff value of 2.8 nmol/L was established from a receiver-operating characteristic (ROC) curve, as discriminating between controls and patients with active disease (specificity 100% and sensitivity 92.3%). CgA levels were higher in functioning than in no functioning tumors (median 55 nmol/L vs 5 nmol/L, p < 0.05). Metastases were present in 83 patients and their CgA levels were significantly higher than levels in the 36 patients without metastases (median 44 nmol/L vs 64 nmol/L, p < 0.0001). CgA levels are strongly correlated with tumor metastatic spread. Sensitivity differed between patients with localized disease (median 6 nmol/L), extensive disease (median 22 nmol/L) and very extensive disease (median 44 nmol/L) (p < 0.001). In conclusion, due to its high sensitivity and specificity, CgA is useful in a newly discovered GEP-NET especially when no abnormal hormone secretion can be demonstrated. CgA levels were significantly higher in functioning tumors than in non-functioning tumors and increased with metastatic spread. If serial evaluation of CgA levels is sufficient for the detection of tumor growth changes remains to be prospectively demonstrated.