Background: There is a dearth of assessments of sign language development in young deaf children. This study gathered age-related scores from a sample of deaf native signing children using an adapted version of the MacArthur-Bates CDI (Fenson et al., 1994). Method: Parental reports on children's receptive and expressive signing were collected longitudinally on 29 deaf native British Sign Language (BSL) users, aged 8-36 months, yielding 146 datasets. Results: A smooth upward growth curve was obtained for early vocabulary development and percentile scores were derived. In the main, receptive scores were in advance of expressive scores. No gender bias was observed. Correlational analysis identified factors associated with vocabulary development, including parental education and mothers' training in BSL. Individual children's profiles showed a range of development and some evidence of a growth spurt. Clinical and research issues relating to the measure are discussed. Conclusions: The study has developed a valid, reliable measure of vocabulary development in BSL. Further research is needed to investigate the relationship between vocabulary acquisition in native and non-native signers.