In 3 pregnant women oestrogen excretion in the urine was very low. The pregnancies were otherwise uncomplicated and the 3 infants, boys, were normal at birth, but later developed ichthyosis of the X-linked inherited type. Histochemically, the placenta in each case showed deficiency in arylsulphatase-type C activity. In all three children the skin showed the same enzyme deficiency. In the skin of 9 other unrelated (adult) patients with proved X-linked inherited ichthyosis vulgaris, arylsulphatase C activity was deficient. Skin from 5 normal adults and 5 normal children showed arylsulphatase C activity to be present. It is concluded that a sulphatase deficiency is a factor in the causation of ichthyosis of the X-linked inherited type.