The pulmonary function of 70 nonsmoking patients with systemic lupus erythematosus (SLE) was evaluated, and the results were compared with those of 70 age- and sex-matched, nonsmoking healthy individuals. Isolated reduction of carbon monoxide diffusing lung capacity (DCO), usually subclinical, was the most commonly detected functional abnormality in the SLE population, whereas it was absent in the comparison group. Isolated small airways disease (SAD) was observed in a relatively high percentage of patients, but not significantly different from that in the healthy people. Restrictive and obstructive patterns were very unusual in the SLE people and absent in the members of the comparison group. Finally, normal lung function was seen in the majority of the latter, but only in one third of the lupus patients. Subclinical respiratory dysfunction, most commonly expressed as diminished DCO, is a common feature of SLE.