The combined occurrence of X-linked steroid sulfatase deficiency of the placenta and X-linked ichthyosis is reported in 6 unrelated boys. Placental steroid sulfatase deficiency was diagnosed on the basis of a very low total estrogen excretion (6 cases), verified prenatally by the dehydroepiandrosterone sulfate (DHEAS) loading test in 4 cases and postnatally by clinical investigations (6 cases) and by biochemical investigations (5 cases). In addition, microsomal arylsulfatase C (MAS) could not be detected in the placental homogenate of the five cases investigated. Lysosomal arylsulfatases were within the normal range. All boys developed well except for X-linked ichthyosis. In the 5 cases investigated the skin biopsy showed the same MAS deficiency histochemically in the granular layer of the epidermis as in the trophoblast cells. The same holds true for the skin of carriers. Steroid sulfatase activity of cultured skin fibroblasts from the boys was almost nil (3 cases). The histochemical technique offers a practical approach in the scientific investigation of keratotic conditions.