Primary pulmonary Kaposi's sarcoma is an uncommon complication in patients receiving immunosuppressive agents and may be difficult to diagnose. We reported a 62-year-old female patient who had received azathioprine and prednisolone for myasthenia gravis for 10 years and was found to have pulmonary Kaposi's sarcoma without cutaneous involvement. The diagnosis based on the presence of typical flame-shaped lesions on chest computed tomography and a typical histopathological finding, with positive immunohistochemical staining for human herpesvirus 8 on lung biopsy specimens. The patient received 2 cycles of systemic chemotherapy with pegylated liposomal doxorubicin. However, bacteremia and cytomegalovirus viremia developed during chemotherapy. The patient ultimately died from respiratory failure 6 months after the diagnosis of Kaposi's sarcoma.