Hunter syndrome is an X-linked recessive disorder. Determination of heterozygotes is of vital importance in genetic counselling. We describe the DNA linkage analysis in 6 Hunter syndrome families and compare it to previous results based on a serum assay for IDS activity. Our results confirm the reliability of the serum assay. The serum test correctly detected 11/12 of the 1st degree relatives tested by the serum assay (6/7 carriers and 5/5 non-carriers). The only case with an apparent false negative result in the serum test was a daughter of a "probable heterozygote" whose serum test was also negative. We suggest that in this family the mother represented a case of germinal mosaicism and her daughter, based on the serum test, was not a carrier. If our interpretation is correct, then the apparent false negative results were correct. It is concluded that in families where the mutation is not known and DNA analysis is not possible due to the lack of informative RFLPs or due to the lack of DNA samples on key individuals, as well as in sporadic cases, the serum test should be applied as an alternative option for heterozygote detection.