Steroid sulfatase deficiency is an inborn error of metabolism characterized during fetal life by decreased estriol production and postnatally by x-=linked ichthyosis. No consistent substrate abnormalities have been found beyond the perinatal period. Utilizing gas chromatography, we found that the cholesterol sulfate concentration was less than 350 micrograms/100 ml plasma in 9 normal adults, 2 subjects with ichthyosis vulgaris and 2 subjects with lamellar ichthyosis. Control red cell membranes had less than 300 micrograms/100 ml erythrocytes. Eight subjects (age 3 months-74 years) with steroid sulfatase deficiency had strikingly elevated cholesterol sulfate levels with means and ranges as follows: plasma - 3,300 micrograms/100 ml (2,700-4,000), red cell membranes- 7,500 (5,200-9,800) Cholesterol sulfate is known to effect membrane stability and the present observations may help to explain the pathogenesis of STS deficiency and x-linked ichthyosis.