Henoch-Schönlein purpura nephritis (HSPN) is considered a form of systemic vasculitis of the small blood vessels with immune pathogenesis. In this disorder, the complement system is recognized as an important mechanism of glomerular injury. The aim of this study is to determine whether the lectin pathway, a novel pathway of complement activation, is related to the pathogenesis of HSPN. Renal biopsy material from 10 patients with HSPN was studied immunohistochemically and examined for a clinicopathologic correlation. Serum levels of complement components, including mannose-binding lectin (MBL), and plasma levels of complement activation products were also evaluated in these patients and compared with levels in patients with immunoglobulin A (IgA) nephropathy or mesangial proliferative glomerulonephritis (GN) without IgA deposition (non-IgA GN). Glomerular deposition of components of the pathway, MBL and MBL-associated serine protease (MASP-1), as well as C3b/C3c, C5b-9, and C4-binding protein (C4-bp), was detected in 8 of 10 patients. Although no significant correlation was found between glomerular deposition of MBL/MASP-1 and histological or clinical findings, the biopsies on all patients with MBL/MASP-1 deposits were performed within 20 weeks from the onset of disease. Levels of plasma C4d, the activation fragment of C4, and C4-bp, a soluble regulatory protein of the pathway, were greater in patients with HSPN than in those with non-IgA GN. However, there was no difference in serum MBL levels between the three groups of patients (HSPN, IgA nephropathy, and non-IgA GN). These results suggest that complement activation through the lectin pathway was involved at the onset of HSPN, and this mechanism might be important in the disease pathogenesis.