The Medical Birth Registry of Norway carries out a population-based surveillance of birth defects on a routine basis. An increased proportion of newborn with Down syndrome was seen among children of young mothers during 1985-1986. Three alternative explanations were considered: a first representing a maternal age specific effect, a second based on a general increase in a subgroup of cases caused by factors not related to maternal age, and a third based on the assumption that a particular birth cohort of young women was carrying a high risk. As 1987 and 1988 showed very low proportions in all age groups, the last explanation was considered less likely. Statistical modeling was used to explore which of the two remaining explanations of the temporary increase was the more likely. The observed changes were compatible with a change in the occurrence of a group of maternal age-independent Down syndrome cases, from 4.59 per 10,000 in 1973-1984 and again in 1987-1988 to a temporary high of 7.68 per 10,000 in 1985-1986. However, the possibility of an age specific change only among young mothers could not be ruled out.