Patients with steroid sulphatase deficiency develop ichthyosis with accumulation of cholesterol sulphate in plasma and in the stratum corneum. The present study was undertaken to determine whether desulphation of the C19 steroid DHEAS is also impaired. The mean plasma concentrations of DHEA and androstenedione were significantly lower for patients than for controls (p<0.02 and 0.001) while the mean concentration of DHEAS was higher (p<0.002). Following intravenous administration of3H-DHEAS, one patient failed to desulphate3H-DHEAS as evidenced by an absence of urinary3H-glucuronides. A second produced normal amounts of urinary3H-glucuronides (indicative of desulphation capacity) in a baseline study but did not desulphate3H-DHEAS following ampicillin treatment to alter gut microflora. A third patient had consistent sulphatase activity with and without ampicillin.