Background: Platelets are heterogeneous in size, density, metabolic, functional, and biochemical properties. Mean platelet volume (MPV) is a measure of the average size of platelet in a blood sample. Aim: We aimed to evaluate the mean platelet volume as a marker for disease activity in children with systemic lupus erythematosus. Materials and Methods: This was a prospective case-control study, which included 50 patients with SLE and 50 age and sex, matched healthy controls. All subjects were subjected to history taking, physical examination and laboratory parameters in active and remission phases of the diseases. Results: The MPV value in the SLE group was significantly higher than control group (9.6 1.3 fL, 9.1 0.57 fL, respectively, p ¼ 0.04). There was a significant increase of weight, blood pressure, urea, creatinine, proteinuria, CRP, ESR,cholestrol, MPV values, SLEDAI-2K scores and significant decrease of HB, albumin, C3, mean platelet volume (MPC) in the active stage than in the remission stage. There was a significant negative correlation between MPV and MPC in active stage of the disease but the correlation was insignificant in remission stage. Conclusion: MPV increased in active phase of patients with SLE and can be an easy, rapid, inexpensive and simple method to assess disease activity in children with SLE. Keywords Systemic lupus erythematosus, platelet volume