Language loss is a concern for the Cherokee Nation in northeastern Oklahoma. As part of language revitalisation efforts, in 2001 Cherokee Nation opened the Cherokee Immersion Preschool in which teachers use only Cherokee throughout the day with their monolingual English-speaking students. Based on a study of the Immersion Preschool that spanned six months, this paper describes the positive backwash effects of assessment on practices of teachers in the programme. Specifically, we examine the role that classroom observation coupled with a formal language assessment--the Cherokee Preschool Immersion Language Assessment--played in identifying undeveloped aspects of the children's Cherokee language skills and targeting specific techniques teachers could use to encourage children to communicate more effectively in Cherokee. The findings suggest that beneficial backwash produced by meaningful and well designed assessments has the potential to positively inform teachers toward improving classroom instructional practice. The Cherokee Immersion Preschool's experience with language assessment has implications for Native American communities implementing early childhood immersion for language revitalisation purposes.