Objective: Behçet’s disease (BD) is a chronic, multisystem disorder that can cause severe morbidity and mortality. Monitoring tools that measure disease activity are required for effective management of BD. We aimed to investigate the association of prognostic nutritional index (PNI) with disease activity in BD.Methods: In this cross-sectional study, we enrolled 88 adult patients with BD and 51 healthy controls. The patients were divided into patients with active and inactive BD according to their disease activ-ities. PNI was calculated using the following formula: 10×serum albumin (g/dL)+0.005×peripheral lymphocyte count (per mm3). To evaluate BD activity, the Behçet Disease Current Activity Form was used. The relations of BD activity with PNI, neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, erythrocyte sedimentation rate, and C-reactive protein were investigated. A receiver operator characteristic curve analysis was used to define the optimum cutoff value of PNI for active BD.Results: A total of 48 patients were classified as having active BD and 40 as having inactive BD. Patients with active BD had lower mean PNI than patients with inactive BD and healthy controls (51.8±4.2, 57.4±2.9, and 56.6±3.6, respectively; p<0.001). In multivariate analysis, PNI was the only independent predictor of BD activity (odds ratio, -0.687; 95% confidence interval 0.548-0.861; p=0.001). The opti-mum cutoff of PNI for active BD was 55.35 with 79.2% sensitivity and 77.75% specificity.Conclusion: PNI was significantly associated with BD activity. PNI may be a useful tool for the assess-ment of disease activity in BD.