Since 1970, epidemiological surveillance of congenital malformations has been carried out in Norway on the basis of data in the nationwide Medical Birth Registry. A separate local registry of cases occurring in the county of Hordaland was established in 1985 to clarify problems inherent in the reporting. This paper deals with double sampling estimators of the prevalence at birth of various malformations, utilizing joint data from two such registries. Particular examples include Down's syndrome, malformations of the central nervous system (CNS), cleft lip or cleft palate, and hypospadias. The methods applied also allow estimation of the probabilities of ascertainment of the different malformations in each registry. Expressions are derived for the variances of the asymptotic distributions of the double sampling estimators, and the possibility of using these values to detect true changes in disease prevalence when ascertainment is uncertain is discussed. The merits of double sampling are assessed comparing the crude estimator based on the Medical Birth Registry alone with the double sampling estimators. Finally, some fully parameterized alternatives to the double sampling models are introduced to evaluate some of the assumptions involved.