AbstractThis study aimed to evaluate the effect of intrathecal (IT) recombinant human arylsulfatase A (rhASA) on magnetic resonance imaging (MRI)‐assessed brain tissue changes in children with metachromatic leukodystrophy (MLD). In total, 510 MRI scans were collected from 12 intravenous (IV) rhASA‐treated children with MLD, 24 IT rhASA‐treated children with MLD, 32 children with untreated MLD, and 156 normally developing children. Linear mixed models were fitted to analyze the time courses of gray matter (GM) volume and fractional anisotropy (FA) in the posterior limb of the internal capsule. Time courses for demyelination load and FA in the centrum semiovale were visualized using locally estimated scatterplot smoothing regression curves. All assessed imaging parameters demonstrated structural evidence of neurological deterioration in children with MLD. GM volume was significantly lower at follow‐up (median duration, 104 weeks) in IV rhASA‐treated versus IT rhASA‐treated children. GM volume decline over time was steeper in children receiving low‐dose (10 or 30 mg) versus high‐dose (100 mg) IT rhASA. Similar effects were observed for demyelination. FA in the posterior limb of the internal capsule showed a higher trend over time in IT rhASA‐treated versus children with untreated MLD, but FA parameters were not different between children receiving the low doses versus those receiving the high dose. GM volume in IT rhASA‐treated children showed a strong positive correlation with 88‐item Gross Motor Function Measure score over time. In some children with MLD, IT administration of high‐dose rhASA may delay neurological deterioration (assessed using MRI), offering potential therapeutic benefit.