Accurate prevalence data for acquired cryptorchidism are currently sparse and systematic prospective studies have not yet been reported. Our aim was to determine the prevalence of testicular ascent in childhood. In a prospective longitudinal population-based child cohort from Copenhagen, Denmark (1997-2007), testicular position was examined according to a standardised protocol in a total of 1072 boys, at birth (n = 1051), at 3 months (n = 983), 18 months (n = 888), 36 months (n = 790) and again once between 4 1/2 and 10 years of age (n = 509). Ascensus testis was defined as ascent of the testis into a cryptorchid position after normal scrotal position at birth. A congenital cryptorchid testis with spontaneous postnatal descent followed by recurrence of cryptorchidism was named recurrent cryptorchidism. Ascensus testis occurred in 0.2%, 0.6% and 0.6% of boys at 3, 18 and 36 months of age respectively. When including recurrent cryptorchidism the prevalence was 0.2%, 1.2% and 0.8% respectively. Ascensus testis accounts for 58% of all cases of cryptorchidism (congenital and acquired) at 18 months, 71% at 36 months and thereafter 69%. Ascensus testis accounts for more than half of cryptorchid testes seen in childhood and occurs in both previously scrotal and cryptorchid testes. We therefore recommend that all boys should have testis position checked regularly during childhood, at least up to 3 years of age.