Cognitive dysfunction (CD) is a poorly understood non-stroke central neurological manifestation in anti-phospholipid syndrome (APS). Brain-derived neurotrophic factor (BDNF) is a neurotrophin that plays an important role in neural plasticity and could potentially be a biomarker of CD in primary APS (PAPS). The aim of the study is to assess CD in PAPS patients and to evaluate its association with clinical data, anti-phospholipid antibodies and serum BDNF levels.This cross-sectional study compared 44 PAPS patients and 20 healthy controls matched for age, gender and education. PAPS patients and controls underwent a standardized cognitive examination. The demographic, clinical and laboratory characteristics of patients were recorded. Serum BDNF was measured by Enzyme Linked Immunosorbent.Fourteen (31.8%) of the 44 patients with PAPS had CD compared with only one (5%) healthy control (P =0.019). PAPS patients presented lower serum BDNF levels when compared with controls (P =0.007). Lower levels of BDNF were associated with CD in PAPS patients (P =0.032). In the univariate analysis, a positive association was found between CD and livedo reticularis, deep vein thrombosis, stroke, seizure, smoking as well as a negative association with Mini Mental State Examination and serum BDNF. According to multivariate analysis, the only independent predictor of CD in PAPS was stroke (OR 137.06; 95% CI: 4.73, 3974.32; P =0.004).CD is commonly reported in PAPS patients; however, its assessment lacks in standards and objective screening tests. The association between CD and low serum BDNF suggests that this neurotrophin can be a promising biomarker for PAPS cognitive impairment.