Magnetic resonance imaging (MRI) and P-31 magnetic resonance spectroscopy (P-31 MRS) provide unique, quantitative data that cannot be obtained from routine laboratory tests. MRI is the method of choice for imaging of muscle abnormalities. It is also a very sensitive technique for localizing nonhomogeneous inflammation in inflammatory myopathies such as dermatomyositis, juvenile dermatomyositis, amyopathic dermatomyositis, polymyositis, and inclusion body myositis. During treatment of inflammatory myopathies, the extent and severity of inflammation may decrease at varying rates, but weakness and fatigue remain serious clinical problems. The metabolic abnormalities detected with P-31 MRS are more persistent and can be used for objective patient evaluation after the disappearance of inflammation and normalization of serum levels of muscle enzymes. With P-31 MRS, biochemical defects are quantitated, including low levels of ATP and phosphocreatine (PCr) and elevated concentrations of ADP and inorganic phosphate (Pi), which may all be related to weakness and fatigue. Thus, MRI and P-31 MRS are useful in assessing the status of patients with inflammatory myopathies during treatment with prednisone and immunosuppressive drugs.