In this report we describe two unrelated young males with severe mental retardation, persisting hypotonia, and constipation. A maternal uncle of one of these two boys died at the age of 18 months and presented the same clinical symptoms. The triad mental retardation, hypotonia, constipation is a characteristic finding in the FG syndrome, an X-linked mental retardation syndrome. At the present time, there is increasing evidence that the FG syndrome-phenotype may be present in different XLMR conditions, e.g. the fragile X syndrome. In addition to the triad severe mental retardation, hypotonia, constipation, the present two male index patients had a characteristic facial appearance with nasal hypoplasia, relative microcephaly and pre- and postnatal overgrowth. The question is raised whether the present two males are examples of a specific entity within the FG-syndrome-like phenotype.