Objective: Pregnancy in patients with systemic lupus erythematosus (SLE) is associated with an increased risk for maternal and fetal adverse outcomes. Defective placentation plays an important role in the pathogenesis of these adverse outcomes. Also, it has been reported that angiogenic proteins are altered even before the onset of clinical manifestations. In this regard, this study aimed to evaluate the usefulness of the soluble fms-like tyrosine kinase-1 (sFlt-1) to placental growth factor (PlGF) ratio in second trimester plasma of patients with SLE for the prediction of adverse outcomes. Methods: This retrospective cohort study included 30 SLE patients and 9 normal pregnancy controls, who were matched for gestational age at sample collection and for maternal age. The study population was divided into three groups: SLE patients with normal pregnancy outcome (Group 1, n=18), SLE patients with adverse outcomes (Group 2, n=12), and healthy controls (Group 3, n=9). The definition of adverse outcomes was at least one of the followings: intrauterine fetal death, preeclampsia, neonatal death prior to hospital discharge, indicated preterm delivery before 36 weeks, and small for gestational age (<5th percentile) at birth. Second trimester maternal plasma was analyzed for sFlt-1/PlGF ratio using high-sensitive multiplex array. Results: Among 30 women with SLE (Group 1 and 2), adverse outcomes developed in 12 (40%) and the rate of intrauterine fetal death, preeclampsia, neonatal death, indicated preterm delivery, and small for gestational age were 25%, 58%, 0%, 67%, and 54.5%, respectively. The median concentration of the sFlt-1/PlGF ratio was significantly higher in Group 2 compared with Group 1 or Group 3. Receiver operating characteristic analysis revealed the predictability for adverse outcomes based on the sFlt-1/PIGF ratio with AUC of 0.847. Conclusion: Our findings indicate that sFlt-1/PIGF ratio in second trimester maternal plasma can be used to predict the development of adverse outcomes in patients with SLE.