Figure 1. Carcinoid tumors derive from primitive stem cells and are generally found in the gut wall. Carcinoids may, however, occur in the pancreas, rectum, ovary, lung, and elsewhere. The tumors grow slowly and often are clinically silent for many years before becoming manifest after metastases have occurred. They frequently metastasize to the regional lymph nodes, liver, and, less commonly, to bone. The likelihood of metastases relates to tumor size. The incidence of metastases is less than 15% with a carcinoid tumor smaller than 1 cm but rises to 95% with tumors larger than 2 cm. These tumors may be symptomatic only episodically, and their existence may go unrecognized for many years. The average time from onset of symptoms attributable to the tumor and diagnosis is just over 9 years, and diagnosis is usually made only after the carcinoid syndrome occurs.