The case was a 65-year-old Japanese male with no immunocompromising diseases or recent medical history. Cigarette smoking history was one pack/day for 25 years. An abnormality on chest X-ray was found three months before hospitalization at an annual health check-up, but he didn't follow recommendations for further testing. He first visited Yodogawa Christian Hospital one month ago because of progressive dyspnea on exertion. The chest X-ray showed diffuse ground-glass opacity (GGO) (Fig. 1a). The chest CT scan revealed diffuse interstitial lung disease consisting of GGO, patterned NSIP (Fig. 2a), and pulmonary function test proved the decline of vital capacity. Laboratory tests showed that increased Krebs von den Lungen-6 and Surfactant protein-D, and autoimmune antibodies were negative (Table). He was diagnosed as NSIP and followed at our outpatient clinic. However, in one month, his dyspnea had progressed further, and body weight had decreased by 3 kg, and he was admitted to our hospital. On admission, vital signs were normal, and oxygen saturation was 96%. Physical examination showed fine crackles on auscultation of the posterior lung fields. There was no superficial lymphadenopathy. The chest CT scan showed that ground-glass opacities had progressed in one month (Fig. 2c). Also, multiple mediastinal lymph nodes enlargements were found (Fig. 2b), and laboratory tests revealed increased interleukin-6 (Table). Therefore, MCD was suspected. Thoracoscopic lung and mediastinal lymph node biopsies were performed, and pathologic findings of the lymph node suggested Castleman disease of mixed hyaline vascular and plasma cell variant (Fig. 3). Pathological findings of the lung showed that the fibrous thickening of interstitium with plasma cells infiltration and the collapse of alveoli (Fig. 4). Human immunodeficiency virus antibody and human herpesvirus-8 antibody were negative, and two major criteria and two minor criteria (increased erythrocyte sedimentation rate and immunoglobulin G) were met for diagnosing iMCD. Tuberculosis, lymphoma, and other mimicking iMCD diseases were excluded by clinical findings, pathological findings and laboratory tests. Then, we diagnosed this case as iMCD preceded by acute progressive diffuse interstitial lung disease. We started treatment with prednisolone 0.7 mg/kg (50 mg/day), and his dyspnea improved within two weeks. Chest images one month later showed improvement (Figs. 1b and 2d). Although prednisolone was tapered by 5-10mg every week and maintained at 15-20mg/body, he did not relapse.