Immunoglobulin A nephropathy (IgAN) is a complex multifactorial disease characterized by genetic factors that influence the pathogenesis of the disease. In this context, an intriguing role could be ascribed to copy number variants (CNVs). We performed the whole-genome screening of CNVs in familial IgAN patients, their healthy relatives and healthy subjects (HSs). In the initial screening, we included 217 individuals consisting of 51 biopsy-proven familial IgAN cases and 166 healthy relatives. We identified 148 IgAN-specific aberrations, specifically 105 loss and 43 gain, using a new statistical approach that allowed us to identify aberrations that were concordant across multiple samples. Several CNVs overlapped with regions evidenced by previous genome-wide genetic studies. We focused our attention on a CNV located in chromosome 3, which contains the TLR9 gene and found that IgAN patients characterized by deteriorated renal function carried low copy number of this CNV. Moreover, the TLR9 gene expression was low and significantly correlated with the loss aberration. Conversely, IgAN patients with normal renal function had no aberration and the TLR9 mRNA was expressed at the same level as in HSs. We confirmed our data in another cohort of Greek subjects. In conclusion, here we performed the first genome-wide CNV study in IgAN identifying structural variants that could help the genetic dissection of this complex disease, and pointed out a loss aberration in the chromosome 3, which is responsible for the downregulation of TLR9 expression that, in turn, could contribute to the deterioration of the renal function in IgAN patients.European Journal of Human Genetics advance online publication, 8 October 2014; doi:10.1038/ejhg.2014.208.