A 74-year-old woman with rheumatoid arthritis was referred for a mass incidentally noted on chest radiograph. Chest CT scan showed cystic lesions in the right lower lobe. The lesion was evaluated as bronchiectasis, and she was followed up. Three years after the initial presentation, the appearance of the lesion had changed significantly and an elevated air-fluid level in the cystic structures was shown on chest CT scan. The preoperative serum progastrin-releasing peptide (proGRP) level was elevated (108.0 pg/ml; normal: <50 pg/ml). Histopathological specimen obtained by standard lower lobectomy confirmed that the lesion was an intralobar pulmonary sequestration. In the resected lobe, there was no malignant finding, but there were neuroendocrine tumorlet cells, which were positive for proGRP. One month after the resection, the serum proGRP level returned to normal. No pulmonary sequestration with high levels of proGRP has been reported, and this is the first case with elevated serum levels of proGRP.