The X-linked FG syndrome is characterised by mental retardation, congenital hypotonia and constipation (which may both be severe), structural anal anomalies and relative macrocephaly in some, and an unusual and characteristic facial appearance. We describe 7 males from 4 families. One had anal stenosis. Two of the mothers and one sister show probable carrier manifestations. The features of the FG syndrome are individually non-specific. We emphasize that the characteristic combination of features is needed to avoid overdiagnosis.