The occurrence of anencephalus as reported to the Norwegian Medical Birth Registry has fallen from 4.9 per 10,000 births in 1967-71 to 2.7 in 1987-90. The decrease is particularly noticeable for births with a gestational length of 28 weeks or more. On the other hand, there has been a significant increase in the number of cases of anencephalus with a gestational age less than 28 weeks. The occurrence of meningomyelocele has remained relatively stable throughout the same period. Ascertainment error could explain these contrasting trends, since cases of anencephalus detected on ultrasound screening may lead to early termination of pregnancy without notification to the Medical Birth Registry. Better routines for notification of malformations are needed to improve the basis for surveillance in Norway.