Sporadic inclusion body myositis (sIBM) is an intractable and progressive skeletal muscle disease of unknown etiology and without effective treatment. While the etiology is still unknown, however, genetic factors, aging, life style, and environmental factors may be involved. Muscle biopsy typically reveals endomysial inflammation, invasion of mononuclear cells into non-necrotic fibers and rimmed vacuoles, suggesting inflammation and degeneration co-exist as part of the pathomechanism. Recent studies implicate amyloid beta accumulation, defects of proteolysis, and immune system abnormalities. The clinical course is slow with chronic worsening. Diagnosis of sIBM is usually made 5 years after onset. Muscle weakness and atrophy in the quadriceps, wrist flexor and finger flexors are the typical neurological findings of sIBM. Dysphagia and asymmetric weakness are often found. Serum creatine kinase is usually below 2,000 IU/L. sIBM is generally refractory to current therapy, such as steroids or immunosuppressants. Elucidation of the pathomechanism of sIBM is the most important to therapy.