The 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 < 0.005), but not to changes in total IgG levels. B cells expressing high levels of CD38 spontaneously produced IgG class anti-dsDNA in vitro. Persistence of activated B cells during periods with clinically quiescent disease in SLE seems to underly hypergammaglobulinaemia but not anti-dsDNA production. Prior to clinical disease activity, further activation of T and B cells occurs, which is paralleled by rises of anti-dsDNA but not of total IgG. This suggests that the production of anti-dsDNA is a T cell-dependent antigen-driven process, which is independent of the polyclonal activation of the immune system inherent to the disease.