Objectives: To evaluate the safety and efficacy of weekly treatment with human recombinant N-acetylgalactosamine 4-sulfatase (rhASB) in humans with mucopolysaccharidosis type VI (MPS VI). Study design: An ongoing Phase I/II, randomized, two-dose, double-blind study. Patients were randomized to weekly infusions of either high (1.0 mg/kg) or low (0.2 mg/kg) doses of rhASB. Six patients (3 male, 3 female; age 7-16 years) completed at least 24 weeks of treatment, five of this group have completed at least 48 weeks. Results: No drug-related serious adverse events, significant laboratory abnormalities, or allergic reactions were observed in the study. The high-dose group experienced a more rapid and larger relative reduction in urinary glycosaminoglycan that was sustained through week 48. Improvements in the 6-minute walk test were observed in all patients with dramatic gains in those walking <100 meters at baseline. Shoulder range of motion improved in all patients at week 48 and joint pain improved in patients with significant pain at baseline. Conclusions: rhASB treatment was well-tolerated and reduced lysosomal storage as evidenced by a dose-dependent reduction in urinary glycosaminoglycan. Clinical responses were present in all patients, but the largest gains occurred in patients with advanced disease receiving high-dose rhASB.