OBJECTIVE: To test the hypothesis that mycophenolate mofetil (MMF) with prednisone provides better control of myasthenic weakness than prednisone alone in the initial management of generalized myasthenia gravis (MG). METHODS: Eighty immunosuppression naïve subjects with mild to moderate generalized, acetylcholine receptor positive MG at 13 centers were randomized to 2.5 g/day MMF plus 20 mg/day prednisone (n = 41) or placebo plus 20 mg/day prednisone (n = 39) and followed in a double-blind fashion for 12 weeks. Subjects over 18 years of age were included if judged to be candidates for immunosuppression; excluded were those with thymoma or severe oropharyngeal or respiratory muscle weakness. The primary measure of efficacy was change in the quantitative MG (QMG) score from baseline to week 12. Study completers could take open-label MMF for an additional 24 weeks, while prednisone was reduced to the minimally effective dosage. RESULTS: The mean change in QMG score was similar in the treated (-4.4 +/- 5.1) and placebo (-3.6 +/- 5.0) groups (p = 0.71). The dosage of prednisone was reduced by a similar amount in both groups during the open-label phase. Subjects tolerated the study drug well, without unexpected adverse events. CONCLUSIONS: This study demonstrated no benefit of mycophenolate mofetil (MMF) with 20 mg/day prednisone compared to 20 mg/day of prednisone alone after 12 weeks. This may be due to greater than predicted benefit from the prednisone dosage used, the short duration of the study, or the absence of any benefit of MMF in this population of patients with myasthenia gravis.