The objective of this study was to determine the morphologic changes of the levator ani muscle of patients with pelvic organ prolapse and stress urinary incontinence. Histological and histochemical analyses of the biopsy specimens of the levator ani muscle obtained from patients with stress urinary incontinence (SUI), pelvic organ prolapse (POP), and a control group were performed. The striated muscle-positive biopsy rate was 26.7% in the SUI group, 15.8% in the POP group, whereas it was 100% in the control group. The diameters of types I and II fibers decreased significantly with age and menopausal time in the control group. Splitting or fragmentation of fibers with red granules, which are called ragged-red fibers, were found in the SUI group. The diameters of levator ani muscle fibers in the control group were significantly larger than those in the SUI group (p=0.034<0.05). The degenerative change in histology and decrease in relative number of levator ani muscle might be associated with women suffering from SUI.