Multilingualism can bring about various positive outcomes to typically developing children. Its effect on children with language difficulties is not yet clear.The aim of this study was to examine the effects of multilingual learning as a medium of instruction (MOI) on first language (L1) acquisition of children with language disorders (LD).Nineteen Cantonese-speaking students aged 5;8-6;8 who were diagnosed with LD were recruited from a school that used Putonghua (an alternative Chinese dialect) as the MOI when learning Chinese language and were compared with 18 age-and-gender-matched Cantonese-speaking students with LD from a school that used Cantonese as the MOI when learning Chinese language. All the students also learned English (L2) as a subject at school. Proficiency in Cantonese was tested at the beginning and the end of the semester in Grade One in terms of: (1) grammar, (2) expressive vocabulary, (3) auditory textual comprehension, (4) word definition and (5) narration.Mixed-model ANOVAs revealed an effect of time on language proficiency indicating positive gains in both groups. Interaction effects between time and group were not significant. There was a trend that children learning Putonghua showed slightly more improvement in auditory textual comprehension.Proficiency gains were similar across groups. The study found no evidence that a multilingual learning environment hinders the language proficiency in L1 in students who have LD.