The FG syndrome is an X-linked recessive mental retardation syndrome. Ten patients are reviewed with special emphasis on the natural history of the intellectual development, constipation, and the prognosis for growth and behaviour. Six out of 10 patients are still macrocephalic, and there is no evidence for a specific growth pattern with respect to height. The degree of mental retardation is is usually severe. The behaviour is characteristically friendly, sociable and over-talkative, with periodic aggression. Six patients have seizures. A characteristic progression seems to occur from congenital hypotonia with joint hyperlaxity at birth, to joint contractures with apparent spasticity and unsteady gait later in life. The constipation was a temporary problem in five cases. The cowlick and the fetal pads persist and are important, but not specific, for the diagnosis.