An 18-year-old male with an XYY karyotype is reported with short stature, normal intelligence and normal personality, in contrast to the XYY syndrome which can be characterized by tall stature, mental subnormality and aggressive behaviour. The patient, in addition, had exostoses of the tibiae bilaterally and type E brachydactyly; this association has not previously been described in patients with the XYY karyotype.