Abemaciclib, a cyclin-dependent kinase (CDK) 4/6 inhibitor, is an anti-breast cancer drug. Cases of severe interstitial pneumonia, including fatal cases, supposedly induced by this drug, have occurred in Japan. However, to date, no case has been reported in the medical journals. We present the case of a 53-year-old female who was treated for right breast cancer for 8 years; initial treatment included radiation therapy. Abemaciclib therapy was initiated 6 months ago because of uncontrolled metastases. The patient subsequently developed dyspnea and was taken to the emergency room. Thoracic imaging revealed right lung-dominant ground-glass opacities, patchy shadows, and traction bronchiectasis. Furthermore, serum KL-6 levels were markedly elevated. Abemaciclib therapy was discontinued after admission, and steroid-pulse therapy was initiated. Thereafter, dyspnea considerably improved, pulmonary shadowing resolved, and KL-6 levels decreased markedly. However, the patient died from progression of hepatic metastases. Thus, close collaboration between breast surgeons and pulmonologists is necessary when using CDK4/6 inhibitors.