OBJECTIVE: Late-onset Pompe disease is a progressive, debilitating, and often fatal neuromuscular disorder resulting from the deficiency of a lysosomal enzyme, acid alpha-glucosidase. This extension study was conducted to determine the durability of the efficacy and safety of alglucosidase alfa observed over a period of 78weeks in the Late-Onset Treatment Study (LOTS). METHODS: Patients who completed the LOTS study were eligible for this open-label extension study and received alglucosidase alfa 20mg/kg biweekly for an additional 26weeks. The primary efficacy assessments were the distance walked during a 6-minute walk test and the percentage of predicted forced vital capacity in the upright position. Data are reported as change from patient's original LOTS baseline for each measure. RESULTS: The benefit of alglucosidase alfa treatment observed in LOTS at Week 78 was, in general, maintained at Week 104. The mean increase in distance walked measured 28.2+/-66.5m from LOTS baseline to Week 78 and 21.3+/-78.0m from LOTS baseline to Week 104. The mean change from baseline in percentage of predicted forced vital capacity was 1.3%+/-5.7% from LOTS baseline to Week 78 and 0.8%+/-6.7% from LOTS baseline to Week 104. Treatment-related adverse events were mainly infusion-associated reactions observed in 35% of patients. No deaths or anaphylactic reactions were observed during the extension study. CONCLUSIONS: The LOTS Extension study showed that patients treated with alglucosidase alfa for up to 104weeks maintained the improved walking distance and stabilization in pulmonary function observed in the first 78weeks of alglucosidase alfa therapy.