The objective of this study is to determine whether serum concentrations of B cell activating factor from the tumour necrosis factor family (BAFF) and/or a proliferation-inducing ligand (APRIL) are associated with clinical manifestations of systemic lupus erythematosus (SLE). Methods BAFF and APRIL concentrations were quantified using a commercial ELISA in serum samples obtained at the time of clinical assessment in 98 patients, and on 245 samples from 75 of these patients followed prospectively. Results Serum BAFF was significantly increased, and APRIL decreased, in patients with either renal or central nervous system (CNS) lupus. In contrast, in cross-sectional analysis, there was no correlation between disease activity (SLEDAI-2k) and serum BAFF or APRIL. In longitudinal follow-up, there was no association between changes in serum BAFF or APRIL and changes in SLEDAI-2k, or between baseline serum BAFF or APRIL and subsequent changes in SLEDAI-2k. However, between-visit changes in BAFF were significantly different in patients with increases in SLEDAI-2k?=?4, compared to patients whose SLEDAI-2k did not change. Conclusions Although neither serum BAFF nor APRIL correlated with disease activity in the overall population, elevated serum BAFF and reduced APRIL may be markers of renal and CNS disease in SLE patients.