Objective. To characterize abnormalities in magnesium levels in the muscles of patients with dermatomyositis (DM) and juvenile dermatomyositis (JDM) and to evaluate the beneficial effects of prednisone and immunosuppressive therapy in elevating free magnesium (Mg 2؉ ) and ATP-bound magnesium (Mg-ATP).Methods. The study groups consisted of 12 adult patients with DM and 10 juvenile patients with JDM. The 2 control groups were 11 normal adults and 6 healthy children. Levels of total ATP in the quadriceps muscles of the subjects were determined during rest, exercise, and recovery, using noninvasive P-31 magnetic resonance spectroscopy (MRS). Concentrations of the biologically active free Mg 2؉ and the enzymatically active Mg-ATP complex were determined from the spectroscopy data by calculation of the chemical shifts of the -phosphate peak of ATP.Results. Mg-ATP levels in DM and JDM myopathic muscles were at least 37% lower than those in normal muscles during rest, exercise, and recovery from exercise (P < 0.0005). Free Mg 2؉ levels were normal in DM and JDM myopathic muscles at rest, but were significantly lower than control values during exercise and recovery (P < 0.029 and P < 0.005 for DM and JDM, respectively). Prednisone and immunosuppressive therapy partially reversed the magnesium abnormalities, as evidenced by elevation of the levels of Mg-ATP and free Mg 2؉ .Conclusion. Low levels of Mg-ATP and free Mg 2؉ are concordant with weakness and fatigue observed in DM and JDM patients. Immunosuppressive therapy alleviates, in part, the magnesium deficits in the diseased muscles. Therefore, Mg-ATP and free Mg 2؉ may play a significant role in the pathophysiology of these diseases.Dermatomyositis (DM) and juvenile dermatomyositis (JDM) are similar yet distinct autoimmune diseases characterized by an erythematous rash and severe proximal muscle weakness. The classic diagnoses of DM and JDM are based on the findings of clinical evaluation, elevated serum levels of muscle enzymes, abnormal electromyographic results, and muscle inflammation on biopsy. More recently, magnetic resonance imaging (MRI) and P-31 magnetic resonance spectroscopy (MRS) have been employed as noninvasive techniques for the evaluation of patients with adult and juvenile DM (1-5).MRI has proven to be a reliable indicator of the extent and severity of muscle inflammation. P-31 MRS monitors metabolic status by determining the levels of the high-energy phosphate compounds ATP and phosphocreatine (PCr), which are required for muscle contraction. These compounds are greatly reduced in the affected muscles of patients with DM and JDM (1,4-6). During exercise and recovery, these patients demonstrate inefficient utilization and regeneration of ATP and PCr (1,5,6). Since MRI and MRS are noninvasive procedures with no known side effects, these techniques may be used to quantitatively evaluate the patients' responses to therapy on a longitudinal basis (7).In the present study, we used P-31 MRS to investigate the abnormalities in magnesium levels in the diseas...