This comparative child-psychiatric study of a group of three-year-old children of low birth weight and a control group of full-term children had its origins in the co-operation which exists between the department of paediatrics and the department of child psychiatry in Sophia Children's Hospital in Rotterdam. Thanks to an increasing paediatric competence which finds expression in the highly specialized neonatal intensive care unit, the low-birth-weight child has an ever better prognosis as regards chances of surviving without serious physical sequelae (leaving aside the group with extremely low birth weights). The "Lancet" article of March 1971 entitled "Changeing prognosis for infants of very low birth weight" (Rawlings et al.) is, as it were, the exponent of this great medical progression. Alongside this, clinical experience had already taught paediatricians and child psychiatrists, via a lot of casuistics, that the overall development of the low-birthweight child- its development not merely from a somatic but also from an affective and cognitive viewpoint- is not always satisfactory. This gave rise to the question whether, from the standpoint of developmental psychology, children of low birth weight constituted a risk group. In order to acquire more comprehensive information than was to be found in most of the available literature, it was decided to undertake a combined intelligence and child-psychiatric study. The study has been restricted to a group of children tested at the age of three years. We are aware that this has its limitations: some researchers are more interested in development during the first year of life; others prefer to base their opinions on the low-birth-weight child during or after puberty, because it is only in adolescence that identity and character are definitively established. This study, therefore, claims to do no more than answer the following question: based on the group investigated, how far has the low-birthweight child come at three years of age with his developmental task compared with his full-term contemporary, and what does he have to contend with in terms of vulnerability?