In the last decade there has been a significant increase in the proportion of XYY males detected prenatally, mostly as a fortuitous finding. It is of utmost importance to obtain a clear idea of the developmental profile of boys with karyotype 47,XYY and of possible problem areas during further development in order to inform the parents correctly during pregnancy and to provide an adequate surveillance later on. In this study we observed 38 XYY males, of which 12 were diagnosed prenatally. We found that these patients are at considerably increased risk for delayed language--and/or motor development. From birth on, weight, height and head circumference are above average values. The majority attends kindergarten in the normal education circuit although in 50% of the cases psychosocial problems are documented. From primary school age on, there is an increased risk for child psychiatric disorders such as autism. Moreover, although normally intelligent, many of these boys are referred to special education programmes.