To investigate the usefulness of low-dose FK506 for the treatment of myasthenia gravis (MG), we treated 19 patients with generalized MG in a 16-week open clinical trial of FK506 (3-5 mg/day). At the end of the trial, total MG scores (range: 0-27 points) improved by 3 points or more in 7 of 19 patients (37%), and activities of daily living (ADL) scores (range: 0-6 points) also improved by 1 point or more in 8 of 19 patients (42%). Nine of 19 patients (47%) showed improvement in either MG or ADL scores. Significant reduction of anti-acetylcholine receptor antibody titers and interleukin 2 production were observed at the end of this study. Minor but commonly observed side effects were an increase in neutrophil count and a decrease in lymphocyte count. No serious adverse events such as renal toxicity or diabetes mellitus were observed during the 16-week treatment period. FK506 could safely serve as an adjunct to steroid therapy for MG at low dosage.