Purpose: To evaluate the safety and efficacy of miglustat in patients with G M2 gangliosidosis. Methods: A randomized, multicenter, openlabel, 12-month study involving patients aged 18 years or older, randomized 2:1 to miglustat (200 mg TID) or "no miglustat treatment." This study was followed by 24 months of extended treatment during which all patients received miglustat. Primary efficacy endpoints were change in eight measures of isometric muscle strength in the limbs and isometric grip strength, evaluated at baseline, and months 12 and 36. Secondary efficacy endpoints included gait, balance, disability, and other neurological assessments. Safety evaluations included adverse event reporting. Results: Thirty patients (67% male, age range 18 -56 years) with late-onset Tay-Sachs disease were enrolled; 20 were randomized to miglustat and 10 to "no miglustat treatment." Muscle and grip strength generally decreased over the study period. No differences were observed between the two groups in any efficacy measure, either during the 12-month randomized phase or the full 36 months. The most common treatment-related adverse events were decrease in weight and diarrhea. Conclusion: Miglustat treatment was not shown to lead to measurable benefits in this cohort of patients with late-onset Tay-Sachs disease. The observed safety profile was consistent with that of the approved dose (100 mg TID) in type 1 Gaucher disease. Genet Med 2009:11(6):425-433.