Objective -- Single nucleotide mutations in the collagen type III (COL3A1) gene cause the vascular type of the Ehlers-Danlos syndrome (vEDS). Patients with this autosomal dominant disorder have a shorter live expectancy due to ruptured vessels and hollow organs. We have chosen a heterozygous Glycine substitution, the most common mutation causing vEDS, to test the effectiveness of allele specific RNA interference (RNAi) for specific silencing of the mutant allele in order to eliminate the mutated phenotype. Methods and results -- We systematically identified small interfering RNAs (siRNAs) that discriminate between wildtype and mutant COL3A1 allele by a luciferase reporter gen assay. The three best working siRNAs were further tested in primary fibroblasts from a normal donor and a vEDS patient. The best discriminative siRNA had the mutation at position 10 Treatment of fibroblasts with this very siRNA resulted in about 90% silencing of the mutant allele without affecting the wild-typ allele. DDIT3, which is up regulated in the unfolded protein response (UPR) of the endoplasmatic reticulum, was increased in vEDS fibroblasts and significantly attenuated after siRNA treatment. This indicates the positive effect of the siRNA on cell stress induced by mutated and thereby unfolded COL3A1. Transmission and immunogold electron microscopy of extracted extracellular matrix from fibroblasts of the vEDS patient revealed no structurally normal fibrils. After siRNA treatment, collagen fibers were detectable like in fibroblasts of normal donors. Collagens I and III were present in these fibrils. Conclusion -- By applying of allele specific siRNAs treatment, we were able to reduce the negative effects of mutated COL3A1 proteins by reducing the UPR and by stimulating the reconstitution of collagen fibers. Thus the use of allele-specific RNAi may be an option to reduce the severity of vEDS.