Introduction Development of hypersensitivity pneumonitis has been reported in association with combination chemotherapy consisting of gemcitabine and paclitaxel. However, diagnosis of this condition is based on computed tomography imaging and correlative bronchoalveolar findings on bronchoscopy. Although transient elevation of the tumor marker CA 15-3 has been reported in patients with interstitial pneumonitis associated with collagen disease, elevation of this marker during drug-induced hypersensitivity pneumonitis has not been reported yet. Case report In this report, we describe a 74-year-old metastatic schwannoma patient with drug-induced pneumonitis secondary to combined gemcitabine and paclitaxel treatment associated with transient elevation of CA 15-3. Management and outcome: The patient responded to steroid treatment with clinical and radiological improvement, and CA 15-3 levels returned to normal within four weeks. Discussion These findings suggest that blood CA 15-3 level has the potential to be used as a marker to monitor drug-induced pneumonitis.