Abstract Aim of the work: To assess serum levels of B lymphocyte stimulator (BLyS) and a proliferation- inducing ligand (APRIL) to determine their correlations with disease activity in pediatric systemic lupus erythematosus (pSLE) and juvenile idiopathic arthritis (JIA) patients. Patients and methods: Twenty-nine pSLE patients and 33 JIA patients were recruited. SLE disease activity was assessed using the systemic lupus erythematosus disease activity index (SLEDAI), while the juvenile arthritis 27 joint disease activity score (JADAS-27) was calculated for JIA patients. Serum samples were assayed for BLyS and APRIL by the enzyme linked immunosorbent assay (ELISA). Results: Serum BLyS and APRIL were elevated in both pSLE and JIA patients compared to controls. Serum BLyS levels correlated with both SLE and JIA disease activity (p=0.042, p= 0.019, respectively) whereas serum APRIL levels correlated positively with JADAS-27 and inversely with SLEDAI (p= 0.001, p= 0.02, respectively). Elevated serum BLyS and APRIL were significantly associated with a lower incidence of nephritis (p= 0.043, p= 0.016, respectively), while elevated serum APRIL significantly associated with negative anti-dsDNA in pSLE patients (p= 0.017). In JIA patients, both serum BLyS and APRIL were significantly associated with the