The diagnosis of mucopolysaccharidosis type II (Hunter syndrome) consisted of four consecutive stages: 1—Assessment of phenotypic characters; 2—Determination of indicators of excretion of urinary glycosaminoglycans and their fractions, primarily heparan and dermatan sulfates; 3—Measurement of the activity of the lysosomal enzyme iduronate-2-sulfatase; 4—DNA diagnostics, including the search for nucleotide substitutions in the IDS gene with an estimate of its frequency and pathogenicity according to the International HMGD Database. Here, IDS is linked to mucopolysaccharidosis type 2.