A case of X-linked ichthyosis diagnosed antenatally by molecular analysis of fetal DNA is described. The diagnosis was made at 16 weeks gestation, following the finding of a maternal serum unconjugated estriol level lower than 0.1 MoM when performing a triple test. Fetal DNA was obtained from cultured amniocytes; two specific regions were amplified by polymerase chain reaction at 5' and 3' ends of the steroid sulfatase (STS) gene on Xp22.3 region. Analysis showed complete deletion of the STS gene on the distal tip of the X-chromosome short arm.