We report a hereditary muscle disorder with features of inclusion body myositis (IBM) in two adult sisters with slowly progressive asymmetrical muscle weakness. The findings of light microscopic and ultrastructural investigations of muscle biopsy specimens were consistent with a diagnosis of IBM. Both patients improved and stabilized on immunosuppressive treatment with corticosteroids and azathioprine. This differentiates our patients from other sporadic and familial cases of IBM. Clinical and histological features are described and compared with those of other previously reported families with IBM.