Skin fibroblasts cultured from patients affected with the Hunter syndrome are deficient in the activity of a protein, named the "Hunter corrective factor," that is required for degradation of dermatan and heparan sulfates. We now show that this factor, purified from human urine, removes about 2% of the sulfate residues from [(35)S]mucopolysaccharide accumulated within Hunter fibroblasts; these groups are derived from "oversulfated" regions of the polymer. Acetone-powder extracts of fibroblasts derived from patients with the Hunter syndrome are deficient in this sulfatase, in contrast to similar extracts from fibroblasts of individuals of other genotype. Hunter corrective factor coupled to alpha-L-iduronidase (or alternatively, mixed extracts from Hurler and Hunter fibroblasts) release iduronic acid from 4-O-alpha-L-sulfoiduronosyl-D-sulfoanhydromannose. We conclude that the Hunter corrective factor is a sulfatase for sulfated iduronic acid residues.