We examined the relationship between peripheral blood and bronchoalveolar lavage (BAL) lymphocyte phenotypes and lung function in 19 patients with SLE, and evaluated their association with disease activity. Lung function assessment showed a mildly restrictive pattern with frequent impairment of transfer factor for carbon monoxide (T1,co) and diffusing capacity of the alveolocapillary membrane (Dm), of late-expiratory airflow rates and with a high prevalence of increased airway resistance. T1,co, Kco and Dm correlated inversely with the numbers of CD8+ cells and CD56+/CD16+/CD3- (NK) cells in BAL. Oxygen radical production, both by stimulated and unstimulated BAL cells and blood polymorphonuclear leucocytes (PMN) was significantly increased in SLE. In comparison with healthy controls, patients with SLE had a lower percentage of CD19+ B cells in the BAL versus an increased percentage of these cells in peripheral blood. HLA-DR expression on CD4+ and CD8+ lung lymphocytes was markedly increased in SLE. Current SLE disease activity was not associated with changes in BAL or peripheral blood lymphocyte phenotypes. Our data suggest that an ongoing cell-mediated immune response is present in the lungs in SLE, particularly involving activated CD8+ T cells and CD56+/CD16+/CD3- NK cells. It is associated with up-regulated local production of oxygen radicals and with impaired pulmonary diffusing capacity. This inflammatory process seems to be independent of general SLE disease activity.