Many demographers and population planners have considered son preference in Asian countries to be a major barrier to reducing fertility. Some of these countries, such as South Korea and China, however, have recently achieved replacement-level fertility, in spite of their strong adherence to son preference. By use of sex-selective abortion and other means, the sex ratio at birth in these countries has been changed at three levels: in the population at large, between families, and within families. At the population level a rising sex ratio has been recorded; at the between-family level an inverse relationship between sex ratio and family size has been observed; and at the within-family level a rapidly rising sex ratio with birth order has been noted. This article presents empirical evidence of these changes and discusses their implications, focusing on the situation in Korea.