The authors present the results they obtained in diagnosing fetal sex in utero by the use of real-time echography. They also have reviewed the other different techniques for the diagnosis and their fallibility. The prediction of fetal sex is possible in 90 p. 100 of echographies carried out routinely. When a very careful well codified technique is used the method becomes highly reliable. In a successive series of 103 pregnancies fetal sex was diagnosed correctly in every case. The authors point out that they diagnose female sex through positive echographic finding and not by a process of exclusion of the male organs.