The patient was a 47-year-old female who had undergone hypophysectomy due to Cushing's syndrome when 46 years old. In April 1992, about 1 month after the operation, chest CT-scan revealed a lesion of 8 x 8 mm in size in the right upper lobe. A high value of serum ACTH was obtained from the A3b wedge blood by cardiac catheterization. She was operated on suspicion of ACTH releasing lung tumor. During the operation, it was histologically diagnosed as lung carcinoid by a frozen section of the lesion. In addition, ACTH of the blood obtained from the right superior pulmonary vein was as high as 37,300 pg/ml, but that in the right main pulmonary artery or the radial artery was 850-970 pg/ml. Right upper lobectomy with dissection of mediastinal lymph nodes was so performed. Post-operative value of serum ACTH was significantly decreased. She was in good course, and is still asymptomatic 8 months after the operation.