Mixed-ethnic children in Japan do not usually acquire the language of their non-Japanese parent. This study looks at their lost opportunity to acquire their minority parent's language through a retrospective investigation of their language experiences from childhood to young adulthood. Transcripts of interviews with ten mixed-ethnic children (ages 18 to 23) were analyzed based on the constructive grounded theory approach (Charmaz 2014 "Constructing grounded theory," 2nd edn. London: Sage). Analysis of codes which emerged from the interviews revealed that family relations, parents' reluctance to speak the minority language and the prioritization of English were some of the factors perceived by the mixed-ethnic children to have contributed to the non-transmission of the minority language. Many of the children described their lost opportunity to acquire the minority language as regretful. Questions posed by Japanese people about their identity and language reminded some participants of their mixed-ethnicity and inability to speak the minority language. These findings suggest that the non-transmission of the minority language has long-term implications on the social and emotional well-being of mixed-ethnic children in Japan.