Late onset Pompe disease (LOPD) or glycogen storage disease type Ⅱ is an autosomal recessive disorder caused by mutations in the GAA gene leading to muscle weakness in all limbs and respiratory difficulty.The purpose of this investigation was to characterize thigh muscles using MR imaging and to examine fatty infiltration and fluid motion in thigh muscles of LOPD patients.Materials and methods MR imaging was performed in thigh muscles in 4 patients.The diagnosis is confirmed by muscle biopsy and genetic analysis.We analysis T1 images to describe the affected muscles with fatty infiltration, STIR images and diffusion-weighted imaging (DWI)to describe the affected muscles with edema.