Treatment of mucopolysaccharidosis type VII (MPS VII) mice with recombinant mouse β-glucuronidase injections has been shown to deliver enzyme to most tissues and to reduce lysosomal storage during the first 6 wk of life. Here we determine the effect of enzyme therapy limited to the first 6 wk of life on survival and growth and follow the subsequent accumulation of lysosomal storage after β-glucuronidase treatment is discontinued. MPS VII mice received 28,000 U of β-glucuronidase iv at weekly intervals from birth to 6 wk of life and were killed at intervals up to 1 y after the last injection. By 29 d after the last enzyme injection, lysosomal storage material in bone was no different in amount than that seen in untreated MPS VII mice. By 85 d, the fixed tissue macrophage system, meninges, and brain glia had also accumulated storage comparable to that seen in untreated controls. One year after treatment, lysosomal storage was similar to that of untreated MPS VII mice in all sites except cortical neurons, where there was still a slight reduction. All treated mice that were not killed earlier, lived longer, were larger, and had milder facial and skeletal deformities than untreated MPS VII mice. These data show that enzyme replacement therapy in MPS VII mice during the first 6 wk of life improves survival and growth. After treatment is discontinued, storage accumulates slowly in the brain and more rapidly in the fixed tissue macrophage system. Whether therapy continued later in life can further improve survival and growth remains to be established.