OBJECTIVE: To assess trends in the prevalence of Down syndrome (DS) from 1986 to 2004 in Victoria, Australia (population approximately 5 million). STUDY DESIGN: The Victorian Birth Defects Register and the Prenatal Diagnosis Database were linked to ascertain all cases of DS. Total and birth prevalence estimates were calculated per year and presented as 3-year moving averages. RESULTS: The total number of cases of DS increased from 113 in 1986 to 188 in 2004. The number of births declined over the first decade of the study, particularly in younger women, but total numbers have fluctuated between 45 and 60 births since 1996. In women under age 35 years, total prevalence was 10/10,000 until 1997 and then increased to 12.5/10,000. In older women, total prevalence increased from 70/10,000 to 90/10,000 in this time frame. Birth prevalence declined at first but remained relatively stable in the later years of the study. The proportion of cases diagnosed prenatally increased from 3% to 60% in younger women. CONCLUSIONS: Our findings demonstrate the continuing need to devote resources to support individuals with DS and their families.