The authors tested the rapid and accurate prenatal and postnatal diagnosis of sex by the method of amplification of specific portions of DNA of sex chromosomes. To ensure a maximum reliability of the diagnosis the authors recommend combined examination of specific sequences from the heterochromatin region of the long arm of the Y chromosome (Yq) and from alphoid satellite sequences of DNA from the pericentromeric portion of chromosome X and Y (Xc and Yc). For further improvement of the reliability the authors recommend to digest the Yc amplification product obtained by means of restriction endonucleases Msp I, Eco Ri and Hini I. Correct assessment of the diagnosis was confirmed in all examined subjects (12) from DNA and chorium.