A 38-year-old female presented with cough and fever. A chest X-ray examination revealed an abnormal shadow in the posteroinferior portion of the left hemithorax, and a laboratory examination showed that the serum carbohydrate antigen 19-9 (CA19-9) level was markedly high (1000 U/ml). A left thoracotomy showed an intralobar pulmonary sequestration of the left lower lobe, and after a left lower lobe lobectomy, the serum level of CA19-9 decreased to normal. Increased CA19-9 activity was detected by immunohistochemistry in the epithelia of bronchioles in the pulmonary sequestration. This communication is the first to report a case of increased activity of CA19-9 in pulmonary sequestration.