Pupils from special school classes affected by supernumerary X-chromosomes (eight girls and eighteen boys) were subjected to a follow-up study over a period of ten years. The risks of social behaviour disturbances, criminality and alcohol abuse were studied, together with that of mental disorder. In the case of the girls no statistically significant differences were found. For the boys, the only difference of statistical significance was that the Klinefelter boys showed a lower frequency of social behaviour disturbances that their controls.