Preferences for male or female children or a balanced number of sons and daughters are common throughout the world. The dominant preference is for male offspring, particularly in less developed countries. Strong son preference is often tempered, however, by a desire to have at least one child of each sex. In more developed countries a balance preference is more common, often together with a strong preference for the first child to be a son. Although it is usually assumed that sex preference can substantially influence fertility, some analysts argue that the effect is negligible. An intermediate position is taken by those who say that sex preference may not have much impact at high fertility levels, but that as average family sizes begin to fall, sex preference will become a more important factor in fertility decisions. Despite the keen interest that has been shown in sex preference, there is surprisingly little empirical evidence of its effects on fertility. Moreover, much of the research in this area is methodologically weak. The measures that have been used in the past have been subject to a number of criticisms that call their results into question. This paper proposes a new measure of the effect of sex preference on fertility that avoids many of the problems inherent in other methods. The measure is based on widely available survey data on the sex composition of children and can be used with any measure of fertility or family planning. It can handle any type of sex preference and does not assume a linear relationship between sex preference and fertility.(ABSTRACT TRUNCATED AT 250 WORDS)