In the county of Fyn (about 500,000 inhabitants) which is a well defined and representative 10% sample of the total Danish population, 15 total birth cohorts were scrutinized for the occurrence of anorectal malformations (ARM). All the patients were followed until the age of 7 or death. In a total of 96,073 births, 29 cases were observed; one concordant monozygotic pair was counted as one case. The study showed the point prevalence at birth to be 3.0 per 10,000; almost the same frequency has been found in Sweden whereas other figures from Europe and North America are lower. Children with ARM have in general a poor prognosis, largely due to the many associated anomalies. In this material, 11 of 12 isolated ARM cases survived until age 7, but five of these had significant handicaps. Out of 17 probands with associated anomalies, only seven survived until age 7, all of them having some handicap. This group of patients spent at least 15 times more days in hospital than an age-matched group from the same region until age 7, and they underwent a considerable number of operations. The study showed an increased frequency of chromosome anomalies among children with ARM.