aerobic exercise in women who have mild to moderate systemic lupus erythematosus (SLE) uncomplicated by organ damage that would limit participation. This is a pilot study to provide additional preliminary data to support a future U01 application. Persistent, excessive fatigue is among the three most debilitating symptoms of SLE and is cited by as many as 50% of patients as the single most debilitating symptom. We have observed, in women with mild SLE, significant relationships among deficits in work capacity and oxygen consumption obtained during treadmill exercise and patient reported measures of fatigue severity. Clinically significant functional aerobic impairment was present in these women, suggesting an underlying pathophysiological limitation that restricted cardiorespiratory capacity to well below that expected to occur as a result of normal deconditioning due to a sedentary lifestyle and lack of physical activity. We have also observed prolonged rest to steady state metabolic transition during even submaximal exercise in women with SLE . At peak exercise, muscle oxygenation deficiency was apparent despite normal increases in central circulatory oxygen delivery to the active muscles. Muscle tissue studies in other laboratories have implicated muscle basal lamina thickening as an oxygen diffusion barrier, which could possibly diminish the rate of tissue oxygen uptake and restrict cardiorespiratory capacity. Aerobic exercise training could improve cardiorespiratory capacity in women with SLE and with that improvement precipitate a reduction in their fatigability, particularly if that fatigability is mediated by impaired cardiorespiratory function.