Sporadic inclusion body myositis (s-IBM) is characterized by late onset of slowly progressive weakness that involves the quadriceps and volar forearm muscles early in the course of the disease. There are hereditary forms of inclusion body myopathy (h-IBM) that histologically resemble s-IBM. The lack of inflammation on biopsy and the different ages at onset and patterns of muscle weakness distinguish s-IBM from h-IBM. We report twin brothers with the typical clinical and histologic features of s-IBM. The occurrence of s-IBM in these twins suggests the possibility of a genetic susceptibility to developing s-IBM.