We report evidence of a dose responsive effect of enzyme replacement therapy in mucopolysaccharidosis type VI cats from birth, at the clinical, biochemical, and histopathological level. Cats treated with weekly, intravenous recombinant human N -acetylgalactosamine-4-sulfatase at 1 and 5 mg/kg, were heavier, more flexible, had greatly reduced or no spinal cord compression, and had almost normal urinary glycosaminoglycan levels. There was near normalization or complete reversal of lysosomal storage in heart valve, aorta, skin, dura, liver, and brain perivascular cells. No reduction in lysosomal vacuolation was observed in cartilage or cornea; however, articular cartilage was thinner and external ear pinnae were larger in some treated cats. Degenerative joint changes were not obviously delayed in treated cats. Skeletal pathology was reduced, with more normalized bone dimensions and with more uniform bone density and trabecular pattern clearly visible on radiographs by 5 to 6 mo; however, differences between 1 and 5 mg/kg dose rates were not clearly distinguishable. At a dose of 0.2 mg/kg, disease was not significantly altered in the majority of parameters examined. Lysosomal storage was present in all tissues examined in the midterm mucopolysaccharidosis type VI fetus and increased rapidly in extent and severity from birth. (J. Clin. Invest. 1997. 99:651-662.)