We determined the strength of correlation among, and responsiveness of, outcome measures used in a multicenter, double-blind, placebo-controlled trial of mycophenolate mofetil in combination with prednisone in myasthenia gravis (MG). The primary efficacy measure was the change from baseline in the Quantitative MG (QMG) score at week 12. Secondary outcome measures included the MG-Activities of Daily Living profile (MG-ADL) and MG Manual Muscle Test (MMT). The measures were collected at baseline and at weeks 4, 8, and 12 in the blinded study and at weeks 16, 20, 28, and 36 in an optional open-label extension. At baseline, the QMG was moderately correlated with the MG-ADL (r=0.55, P<0.0001) and the MMT (r=0.53, P<0.0001), but the correlation between the MG-ADL and the MMT was lower (r=0.30, P=0.007). These findings were similar at weeks 4, 8, and 12. Similar correlations were found among the changes in scores from baseline at weeks 12 and 36. The MMT and MG-ADL appeared to be the most sensitive measures for changes in MG status at weeks 12 and 36. Although a task force has recommended use of the QMG in prospective MG trials, the MMT and MG-ADL appear to be suitable alternatives and offer potential advantages. No special training or equipment is required, and they take less time.