Although the role of single gene and chromosomal disorders in pediatric illness has been recognized since the 1970s, there are few data describing the impact of these often severe disorders on the health of the adult population. In this study, we present population data describing the impact of single gene and chromosomal disorders on hospital admissions of patients aged 20 years and over in Western Australia between 2000 and 2006. The number, length, and cost of admissions were investigated and compared between disease categories and age groups and to hospital admissions for any reason. In total, 73,211 admissions and 8,032 patients were included in the study. The most costly disorders were cystic kidney disease, α-1 anti-trypsin deficiency, hemochromatosis, von Willebrand disease, and cystic fibrosis. Overall, patients with single gene and chromosomal disorders represented 0.5% of the patient population and were responsible for 1.9% of admissions and 1.5% of hospital costs. These data will enable informed provision of health care services for adults with single gene and chromosomal disorders in Australia.