To determine the serum levels of galectin-3 (Gal-3) and galectin-3 binding protein (G3BP) and to evaluate the associations between clinical features and these levels in patients with Behçet's disease (BD).Fifty patients with BD (mean age 40.6 +/- SEM 1.4 years; 21 males, 29 females; 26 active and 24 inactive patients), 20 patients with rheumatoid arthritis (RA), and 20 patients with systemic lupus erythematosus (SLE) were enrolled. Clinical features of BD patients including BD activity and severity over the previous 4 weeks were reviewed and serum levels of Gal-3 and G3BP were determined using enzyme-linked immunosorbent assays (ELISA).Serum Gal-3 levels were significantly higher in total BD patients than in healthy controls (mean +/- SEM, 10.68 +/- 0.93 versus 7.59 +/- 0.48 ng/mL; p = 0.0042 by Student's t-test), and active BD patients had significantly higher levels of serum Gal-3 than inactive patients and controls (13.08 +/- 1.53 in active BD, 8.08 +/- 0.71 ng/mL in inactive BD; p = 0.000039 by one way ANOVA). Although mean G3BP serum levels were not different in total BD patients and controls, active BD patients (6806.63 +/- 468.58 ng/mL) had higher G3BP levels than controls (5421.05 +/- 286.02 ng/mL; p = 0.031 by one way ANOVA). Additionally, serum Gal-3 significantly increased in patients with RA (p = 0.019 by t-test) and SLE (p = 0.00069) and G3BP increased in patients with SLE (p = 0.000012), compared to those in healthy controls. When we analyzed for associations with clinical features over the previous 4 weeks, Gal-3 was associated with orogenital ulcers (p = 0.036 by t-test) and time elapsed from symptom onset (p = 0.032, Pearson's coefficient = 0.314). Serum concentrations of Gal-3 (p = 0.013) and G3BP (p = 0.032) were positively correlated with the BD severity score for the previous 4 weeks. Gal-3 levels were significantly correlated with TNF-alpha (p = 0.048, Pearson's coefficient = 0.281) and G3BP levels were correlated with levels of C-reactive protein (p = 0.021, Pearson's coefficient = 0.329) in total BD patients. In multivariate analysis of all cytokines levels, only Gal-3 was significantly related to BD activity or severity for the previous 4 weeks.These results suggest that serum levels of Gal-3 and G3BP are increased in active BD patients and Gal-3 can be a new biomarker indicating disease activity in BD although their increments are not disease-specific.