Because of congenital deficiency of the superior sacral facet of S1 or the arch of L5 in dysplastic spondylolisthesis, progression of slippage may occur even in the presence of a posterior solid arthrodesis, especially in the patients with high degree of slippage and lumbosacral kyphsis. Therefore, the anterior interbody fusion may be necessary additionally. But the operation is very difficult because of a little supporting structure anteriorly for bone grafting. We have a case of dysplastic spondylolisthesis which showed progressive slippage even though solid posterolateral fusion. On which case we performed additional anterior interbody fsuion by the technique of anterior open wedge osteotomy on the superior dome of sacrum to support the L5 on S1. And this is the case report of the result.