A new concept of classifying neuroendocrine pancreatic tumors based on clinicopathologic patterns was summarized recently. To evaluate the clinical reliability and prognostic specificity of this classification system, 100 neuroendocrine pancreatic tumors were retrospectively categorized as "benign," "uncertain," and "malignant" based on tumor risk factors (size, local invasion and angioinvasion, cell atypia, metastases) and were followed for disease recurrence and progression. Altogether, 71 functioning tumors (insulinoma, gastrinoma, glucagonoma, enterochromaffin-like (ECL)oma, somatostatinoma) and 29 nonfunctioning neuroendocrine pancreatic tumors (NETs) were studied. NETs had an increased risk of malignancy (p < 0.05). Tumor size, gross invasion, and metastases correlated significantly with tumor behavior and allowed us to distinguish between "benign" and "malignant" tumors. About 89% of the tumors < or = 20 mm were "benign," whereas 71% > 20 mm were "malignant" (p < 0.05). In patients with "benign" and "uncertain" neuroendocrine pancreatic tumors, neither recurrence nor progression of disease was seen. About 41% of the patients with "malignant" tumors died of the disease. The 5-year estimated cumulative survival of those with "benign" and "uncertain" tumors was 100% and 52 +/- 10% for those with "malignant" tumors (p < 0.05). Histomorphologic details classifying the behavior of an "uncertain" tumor are known only after initial treatment and definitive histopathologic investigation. Thus this information is of limited clinical help for treatment strategies.