X-linked Hunter syndrome (MPS II) is presently thought to exist in two clinically and genetically distinct forms, mild and severe, which are biochemically indistinguishable. However, two sibships have been studied in which both mildy and severely affected children are present. Therefore, genetic counseling for families with MPS II should be reconsidered to take into account the possibility of heterogeneity within a family in terms of the degree of psychomotor retardation and potential longevity.