Abstract This is the first part of a wider study which attempts to throw iight on the demographic, economic and social factors that have led to dramatic declines in fertility levels in most socialist countries of Eastern Europe during the last fifteen years or so. The present part is concerned with the purely demographic influences, that is mainly with the impact of changes in the age and sex structure of the populations under study, and in nuptiality. The statistical evidence adduced indicates that the observed downward trends in the annual number of births and in crude birth rates are a reflection of genuine changes in attitudes towards family size.