A CENTRAL CONCERN of demographers and population planners has been the impact on family size of parental preferences for the sex of their children, especially in patriarchal developing societies in Asia. If parents continued to bear children until they reached their desired sex combination of children or their desired number of sons, sex preference would be a major barrier to fertility reduction. Sheps (1963) has shown theoretically that the expected average family size would be 3.88 children if couples continued childbearing until two sons were born. In fact, abundant empirical evidence shows a close relationship between sex preference and fertility.' In China and Korea (these designations refer to Mainland China and South Korea, respectively, throughout the article), fertility has recently declined precipitously to the replacement level or even below, in spite of their populations' strong adherence to son preference. In these countries, however, probably to accommodate both sex preference and a small-family norm, a new demographic phenomenon of a distorted sex ratio (number of males per 100 females) at birth is emerging at three levels: in the population at large, between families, and within families. At the population level, a rising trend has been recorded in the sex ratio. At the between-family level, large families have low sex ratios and small families high sex ratios. At the within-family level, a rapidly rising sex ratio is reported with rising birth order, and the sex ratio of last-born children is extremely high. In this article we present empirical evidence of these changes in the sex ratio at birth, focusing on Korea. Then we discuss possible demographic, social, health, and other implications of the changes. The principal means of altering the sex ratio at birth, also known as the secondary sex ratio, are