The effects of corticosteroid administration to eight patients with severe thoracic systemic lupus erythematosus (SLE) were assessed in a prospective study over a mean period of 26 months by serial measurements of respiratory function, ESR, and a clinical score. Initial assessment of respiratory function showed severe restrictive ventilatory defects and impairment of carbon monoxide uptake not wholly attributable to the small lung volumes. ESR and clinical score showed high correlation coefficients with FEV1 in all patients, with VC and TLC in seven patients, and with TLCO in four patients, indicating that changes of respiratory function were reflecting the activity of the disease. This study shows that in patients with severe thoracic SLE it is valid to use serial measurements of respiratory function to assess the response to treatment and that pronounced and sustained improvement of respiratory function can be expected.