The authors describe the first mother-infant pair to complete an on-going, prospective, open-label, Phase 4 trial (ALID 01803, NCT00418821) determining the safety of laronidase enzyme replacement therapy (ERT) in pregnant women with mucopolysaccharidosistype I (MPS I) and their breastfed infants. The mother, a 32-year-old with attenuated MPS I (Scheie syndrome), received laronidasefor three years and continued treatment throughout her second pregnancy and while lactating. A healthy 2.5 kg male wasdelivered by elective cesarean section at 37 weeks. He was breastfed for three months. No laronidase was detected in breast milk. Theinfant never developed anti-laronidase IgM antibodies, never had inhibitory antibody activity in a cellular uptake assay, and always hadnormal urinary glycosaminoglycan (GAG) levels. No drug-related adverse events were reported. At 2.5 years of age, the boy is healthywith normal growth and development. In this first prospectively monitored mother-infant pair, laronidase during pregnancy and breastfeedingwas uneventful.