During a 5-year period (1986 to 1990), 7 patients with thoracic ectopic ACTH-producing tumors (4 pulmonary and 3 thymic) associated with Cushing's syndrome were diagnosed and treated in PUMC Hospital, Beijing. Among the 7 patients, 4 were male and 3 were female, with a mean age of 35.7 years. The duration of Cushing's syndrome before operation ranged from 6 months to 72 months. Histologically all tumors were typical carcinoids. Various amounts of neurosecretory granules were found in the tumor cells of all tumors by electron microscopy. Immunohistochemically all tumors were strongly positive for chromogranin A and NSE. The immunopositivity for ACTH was variable in different tumors. Some tumors were also positive for S-100, calcitonin, HCG alpha and HCG beta. By in situ hybridization technique, overexpression of chromogranin A mRNA was detected in all tumors.