CASE PRESENTATION: A 44-year old woman with a history of SLE, Lupus Nephritis Class IV-V, presented to the Rheumatology Clinic with dyspnea on exertion. She had recently completed 6 cycles of cyclophosphamide induction therapy for lupus nephritis and was receiving hydroxychloroquine, mycophenolate, and prednisone. Physical examination was unremarkable. SLE Disease Activity Index score was unchanged from the previous visit. Imaging of the chest was perfomed to rule out life threatening conditions. Computerized Tomography Angiography failed to reveal parenchymal pulmonary disease or pulmonary artery filling defects. However, pulmonary function testing (PFT) showed a restrictive pattern with impaired diffusion capacity. To evaluate for diaphragmatic paralysis, seated and supine PFTs were performed which did not show Forced Vital Capacity (FVC) changes in supine position. ECG, echocardiogram and nuclear imaging failed to reveal cardiac causes of dyspnea. Afterwards, the differential diagnosis was narrowed to: SLS, myasthenia gravis, and steroid-induced myopathy. CPK level was slightly increased and anti-acetylcholine receptor antibody was negative. Since the patient had no weakness, myopathy was thought to be unlikely. Belimumab was chosen as a steroid sparing agent. The patient reported resolution of dyspnea after initiating therapy. Her exercise tolerance returned to baseline and repeat PFT showed a persistent restrictive pattern with increased TLC.