Mannose-binding lectin (MBL) is a soluble pattern-recognition molecule, which plays a crucial role in the innate immune system and the activation of lectin complement pathway. Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is an autoimmune disease affecting the small vasculatures and is characterized by the alteration of innate and adaptive immunity and complement activation. In this study, we investigated whether serum MBL is associated with disease activity of AAV, which was measured by ELISA. Associations between serum MBL and AAV-specific indices, as well as clinical and laboratory data were assessed using Kendall's tau. Among the 80 patients, 42 (52.5%), 21 (26.3), and 17 (21.3%) patients were classified as microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophilic granulomatosis with polyangiitis (EGPA), respectively. The median values of erythrocyte sedimentation rate, C-reactive protein, and serum MBL were 36.5 (normal range < 20) mm/h, 2.4 (normal range < 8) mg/dL, and 8.6 ng/mL, respectively. The median serum levels of MBL in MPA, GPA, and EGPA patients were 8.4, 9.3, and 8.2 ng/mL. Correlation analysis showed that serum MBL was associated with Birmingham Vasculitis Activity Score (BVAS) (R = 0.169, p = 0.027), but not with other AAV-specific indices and clinical and laboratory data. In addition, serum MBL was significantly associated with the pulmonary manifestation score based on BVAS (R = 0.247, p = 0.001). In summary, among the AAV-specific indices and clinical and laboratory variables analyzed, serum MBL is correlated with BVAS and pulmonary manifestation score based on the BVAS.