 This 2 arm study will provide optional continuation of double-blind treatment with CellCept or placebo, in patients with myasthenia gravis who have achieved good symptom control in study WX17798. Patients who have completed 36 weeks of treatment in study WX17798, with stable prednisone dosing for the last 4 weeks, can continue on blinded treatment with CellCept 1g bid or placebo until the database for WX17798 is locked and unblinded. The anticipated time on study treatment is 3-12 months, and the target sample size is 100-500 individuals. 