Lupus pneumonitis carries high mortality and is a rare manifestation of systemic lupus erythematosus (SLE). However, it is difficult to diagnose and is often mistaken as pneumonia, alveolar haemorrhage, or organizing pneumonia. Previous studies demonstrated that serum anti-Ro antibodies are elevated more frequently in SLE patients with pneumonitis than in those without. We report a 21-year-old female who was newly diagnosed as having SLE with nephritis and who suddenly developed right lung opacity and rapidly progressed to severe hypoxaemia despite the use of broad-spectrum antibiotics. The serum titre of anti-Ro antibody was greater than 240 U/mL. She underwent lung biopsy and lupus pneumonitis was confirmed by the pathological findings. Subsequently, she showed a favourable response to plasma exchange, steroid pulse therapy, and mycophenolate mofetil (MMF) treatment. For SLE patients with pulmonary infiltrates, high degree of clinical suspicion of lupus pneumonitis is required and measurement of serum anti-Ro antibody may help to make the diagnosis.