To study steroid sulfatase acticity in women in the field of obstetrics and gynecology, especially to differentiate carrier women with steroid sulfatase deficiency (recessive X-linked ichthyosis, RXLI) from normal women, steroid sulfatase activity was assayed in peripheral blood leukocytes from normal non-pregnant women, pregnant women, patients with RXLI, carriers of RXLI and in other normal males of different age. Steroid sulfatase activity in pmol/mg protein/min was significantly lower in patients with RXLI than in the other groups and significantly higher in the 3rd trimester pregnant women than in nonpregnant women and adult men. In addition, sulfatase activity was significantly higher in the 3rd trimester pregnant women than in fetuses. However, it was difficult do differentiate carrier women with RXLI from normal women because of considerable overlap between the two groups. The biochemical control mechanism of steroid sulfatase is the subject of further research.