Initially described as a rare MCA/MR syndrome occurring only in boys, due to a recessive mutation on the X chromosome , the FG syndrome (FGS) now emerges as a more common disorder also occurring in girls. Based on over 50 reported cases, FGS is associated with developmental delay (especially speech), hypotonia, postnatal onset relative macrocephaly, prominent forehead, frontal hair upsweep, telecanthus, or ocular hypertelorism, thin vermilion border of the upper lip, relatively short fingers with broad thumbs and halluces, persistent fetal fingertip pads, anal anomalies, and/or constipation. Major malformations are rare, and include pyloric stenosis, anal agenesis, cryptorchidism, hypospadias, and congenital heart defects. Abnormal EEGs and seizures have been reported in almost 70% of patients. Brain MRI shows corpus callosum abnormalities associated with dilatation of lateral ventricles and, less frequently, periventricular nodular heterotopias, mild cerebellar defects, and reduced periventricular white matter. Chiari 1 malformation seems to be frequent. The behavior phenotype appears to be characterized by ADHD, and relatively less developed language, fine motor and executive function skills; whereas visual-spatial abilities seem to be a relative strength. Five candidate loci are already known but no gene identified. We describe 25 patients referred to the Stella Maris Institute for evaluation of DD/MR, and diagnosed as FGS. They were between 2 and 15½ years at the first observation. High resolution banding, FRAXA/FRAXE DNA analysis, and subtelomere FISH analysis were performed in all of them, and all had normal results. Thirteen patients were followedup from 6 months to 9 years. Our report focuses on physical, neurological, developmental findings, and natural history of FGS. Experience with our series of patients suggests that the syndrome may be common, and should be routinely considered in the evaluation of children and adolescents with DD/MR. ß