Factors that determine the clinical course and outcome of patients with gastrointestinal (GI) carcinoid tumors are complex and multifaceted. These include the site of origin within the GI tract, the size of the primary tumor, and the anatomical extent of disease, whether localized, regional, or metastatic to distant sites. The new World Health Organization (WHO) histological classification of endocrine tumors, including carcinoids, represents a significant advance in terms of providing a consistent framework for histopathological interpretation that should facilitate multicenter research on treatment outcomes. Histochemical indicators of a poorer prognosis are the degree of expression of the proliferation protein Ki-67 and the p53 tumor suppressor protein. Adverse clinical indicators are the malignant carcinoid syndrome, carcinoid heart disease, and high concentrations of the tumor markers, urinary 5-HIAA and plasma chromogranin A.