SUMMARYThe objective was to serially analyse T and B cell activation in relation to autoantibody production during the development of relapses in SLE. In a prospective study we serially analysed, by flow cytometry, T cell activation in relation to B cell activation and anti-dsDNA production in quiescent SLE and during the development of a clinical relapse. In addition, we related changes in T and B cell activation to changes in levels of anti-dsDNA and total IgG. During periods with clinically quiescent disease, the expression of activation markers on T cells (IL-2R and HLA-DR) and B cells (CD38) was persistently higher in SLE than in healthy controls (P < 0 . 001). Percentages of CD20 CD38 B cells were related to levels of total IgG (P < 0 . 02), but not to levels of anti-dsDNA. Development of disease activity was paralleled by an increase in the percentages of CD4 T cells (P < 0 . 005) and CD20 CD38 B cells (P < 0 . 001), which were interrelated. Increases in B cell activation were related to increases in levels of anti-dsDNA (P