Sporadic inclusion body myositis (IBM) is the most frequently acquired inflammatory myopathy of late adult life, yet its diagnostic criteria and pathogenesis remain poorly defined. Because effective treatment is lacking, research efforts have intensified to identify specific markers for this debilitating disorder. In this study, proteomic analysis of 4 cases of sporadic IBM was compared with 5 cases of inflammatory myopathy without clinicopathologic features of IBM to distinguish the IBM-specific proteome. Proteins were separated by 2-dimensional polyacrylamide gel electrophoresis and profiled by mass spectrometric sequencing. Expression of most proteins remained unchanged; however, 16 proteins were upregulated and 6 proteins were downregulated in IBM compared with cases of non-IBM inflammatory myopathy. These IBM-specific proteins included apolipoprotein A-I, amyloid beta precursor protein, and transthyretin, which have been associated with amyloidosis; superoxide dismutase, enolase, and various molecular chaperones indicate perturbations in detoxification, energy metabolism, and protein folding, respectively. The IBM-downregulated proteins mainly serve as carriers for muscle contraction and other normal muscle functions. We further applied Western blot and immunohistochemistry to verify the proteomic findings. This study validates proteomics as a powerful tool in the study of muscle disease and indicates a unique pattern of protein expression in IBM.