Children with Down syndrome have an increased risk of cryptorchidism, but the reported incidence is unclear. In a proportion of these children, the testes are within the scrotum at birth but later appear to have ascended to an ectopic position. Records of patients diagnosed with trisomy 21 who had surgery for undescended testes in two tertiary paediatric centres over a 10-year period were examined. Information on liveborn males with Down syndrome was obtained from the Victorian Genetic Registry, and then the incidence of congenital and acquired undescended testes was determined. The incidence of undescended testes in Down syndrome was found to be 6.52% (24/368), with 4.35% (16/368) being acquired undescended or ascending testes. In conclusion, there is an increased incidence of cryptorchidism in Down syndrome; in particular, there is a significant proportion of acquired undescended testes.