Previous studies have indicated that the fragile X [fra(X)] gene does not show full penetrance (mental impairment) in carrier females or "carrier" males. The phenomenon of non-expressing carrier males distinguishes the fra(X) syndrome from all other known X-linked disorders. Moreover, penetrance of the fra(X) gene apparently does not show random distribution within fra(X) families, but seems to be reduced in sibs of normal transmitting males (NTM's). The availability of many large multigeneration fra(X) families, studied by cytogenetic and DNA analyses, enabled us to refine the estimates of the penetrance. From our data we conclude that the penetrance in daughters of carrier females is determined by the mental status of the mother. In sons of carrier females, the observed penetrance appears to be influenced by the grandparental origin of the gene as well as by the mental status of the mother. However, in contrast with the average penetrance, we observed a strongly reduced penetrance of the fra(X) gene in brothers (14%) and sisters (21%) of NTM's. These findings have profound implications for genetic counseling.