BACKGROUND Mesangial IgA1 deposition is characteristic of IgA nephropathy (IgAN). Structural abnormalities of the IgA1 glycoprotein may play a key role in its mesangial deposition, particularly the recently described abnormalities of O-glycosylation of the IgA1 hinge region. The mechanism of abnormal O-glycosylation has not yet been elucidated; it is not clear whether there is an alteration in the amino acid sequence of the hinge region, modifying the number of O-glycosylation sites available, or whether there is a post-translational defect in the glycosylation process. METHODS The O-glycosylation of serum IgA1 from a series of patients with IgAN and matched controls was assessed by lectin binding assay. We then used dideoxy-sequencing of the PCR-amplified hinge region of the alpha1 heavy chain gene to compare the hinge region nucleotide sequence in IgAN and controls. We also compared cDNA transcripts of alpha1 hinge region mRNA to look for evidence for a transcriptional abnormality in IgAN. RESULTS Lectin binding assays confirmed that the IgAN subjects used in this study did indeed display the previously reported abnormality of IgA1 O-glycosylation. However, the hinge region nucleotide sequence of the alpha1 gene was identical in IgAN and controls. There was also no difference in the sizes of cDNA transcripts of hinge region mRNA from patients with IgAN and controls. CONCLUSIONS We found no evidence for any nucleotide sequence alteration or transcriptional abnormality of the alpha1 hinge region in IgAN, and we conclude that the O-glycosylation defect is post-translational.