Doctors, psychologists, physiotherapists, speech therapists and others whose work is concerned with assessing the performance of children known or suspected to be suffering from a variety of handicaps are becoming increasingly interested in the possible antecedent factors. Among the perinatal factors which are considered to be possibly relevant, low birthweight is prominent: but it is important to know whether the effects of being born too soon carry the same implications as those being born too small, and what may be the effect of associated biological and social factors. This knowledge is important not only as a contribution to the diagnosis, but also as a help in deciding about prognosis and the likelihood of response to a particular form of treatment.