In 182 patients with systemic lupus erythematosus (SLE), oral mucosal ulceration occurred in 47 patients (26%), was usually painless (82%), and most often involved the hard palate (89%). Oral ulceration was associated with an increase in overall clinical activity, although this was not accompanied by significant changes in the levels or titers of C3, anti-DNA antibodies, and anti-nuclear antibodies. Necrotizing vasculitis was not observed. Microscopic changes were similar to the skin lesions of SLE and immunoglobulin and complement were found in both the basement membrane and blood vessel walls.