A mean prevalence for KS of 152 per 100,000 male births was estimated from newborn screening programs in the 1960s and 1970s in several countries, including Denmark, the United States, Canada, Japan,and the United Kingdom. Despite this high frequency, and features such as small testicles in adulthood, it has been estimated that less than 10% of the estimated number of affected fetuses are detected prenatally, and only 26% of live-born cases are diagnosed postnatally. A birth prevalence for KS of 153 per 100 000 males in Denmark has been estimated using population information and adjusting the prenatal prevalence for maternal age, as KS is an incidental finding of prenatal karyotype tests that are more commonly performed in older mothers. Comparison with postnatal diagnoses confirmed that only 25% of KS cases are detected. The low diagnosis rate suggests most males with KS will not receive potentially beneficial treatments, especially androgen therapy. Most adult diagnoses occur during fertility assessment, beyond the ideal point for intervention. Detection in childhood and timely intervention may be essential for optimal medical and psychosocial outcomes in adulthood.