According to enrollments in schools for the deaf and data from the national census and neonatal hearing screening programs, the incidence of severe and profound childhood deafness in Australia is, and has been, less than commonly assumed. Factors implicated include improved medical care, mainstreaming, cochlear implants, and genetic science. Data for the United States, Britain, and other developed countries seem consistent with those for Australia. Declining prevalence and incidence rates have immediate implications for sign-based education, teacher-of-the-deaf training programs, and educational interpreting. There are also serious consequences for research, documentation, and teaching regarding Australian Sign Language (Auslan), and for the future viability of Auslan. Prompt action is essential if a credible corpus of Auslan is to be collected as the basis for a valid and verifiable description of one of the few native sign languages in the world with significant attested historical depth.