Carbohydrate antigen 19-9 (CA19-9), also known as sialyl Lewis, is well-known as a tumor marker with a moderate sensitivity and specificity for pancreatic cancer and biliary tract tumors. In addition, serum CA19-9 level elevation is sometimes observed in patients with other tumors or benign diseases. However, the relationship between CA19-9 and chronic bronchitis might not be familiar to physicians. Here, we report a case of chronic bronchitis with elevated serum CA19-9 levels that decreased after treatment with a macrolide, erythromycin. A 71-year-old woman presented to a clinic complaining of chronic cough lasting 6 months. The chest X-ray showed consolidation in the left lower lung field, therefore infectious diseases and lung malignancy including metastatic tumor were suspected. The measurement of serum tumor makers revealed that the CA19-9 level was high enough to suggest the existence of malignancy. The patient was then referred to our hospital for further systemic examinations; however, there was no apparent evidence of malignancy including in the gastrointestinal tract, and the patient was diagnosed with chronic bronchitis. After initiation of treatment with erythromycin for the chronic bronchitis, the serum CA19-9 level gradually decreased and the respiratory symptoms eased. This present case indicates that serum CA19-9 levels might not only serve as a tumor maker related to gastrointestinal cancer but also as a marker of intense airway inflammation.