Background Patients with infantile-onset Pompe disease (IOPD) can be treated by recombinant human acid alpha glucosidase (rhGAA) replacement beginning at birth with excellent survival rates, but they still commonly present with speech disorders. This study investigated the progress of speech disorders in these early-treated patients and ascertained the relationship with treatments. Methods Speech disorders, including hypernasal resonance, articulation disorders, and speech intelligibility, were scored by speech-language pathologists using auditory perception in seven early-treated patients over a period of 6 years. Statistical analysis of the first and last evaluations of the patients was performed with the Wilcoxon signed-rank test. Results A total of 29 speech samples were analyzed. All the patients suffered from hypernasality, articulation disorder, and impairment in speech intelligibility at the age of 3 years. The conditions were stable, and 2 patients developed normal or near normal speech during follow-up. Speech therapy and a high dose of rhGAA appeared to improve articulation in 6 of the 7 patients (86%, p = 0.028) by decreasing the omission of consonants, which consequently increased speech intelligibility ( p = 0.041). Severity of hypernasality greatly reduced only in 2 patients (29%, p = 0.131). Conclusion Speech disorders were common even in early and successfully treated patients with IOPD; however, aggressive speech therapy and high-dose rhGAA could improve their speech disorders.