In this paper we describe the growth of a national sample of premature children who have been followed up from birth to 4 years and who, throughout this period, have been contrasted with a closely matched group of children born at term. This is the second of three papers about these children; the first (Douglas and Mogford, 1953) Illingworth (1939) found that a high proportion of the premature children attending an out-patient department were stunted and, in a later paper (Illingworth, Harvey and Jowett, 1950), showed that there is a direct relationship, that is, not limited to those children normally regarded as premature, between birth weight and subsequent physical development. Asher (1946) concluded that many premature children remain smaller than their fellows at least up to the age of 6 years. Drillien (1948) regarded the physical retardation of her sample of premature children as being a reflexion of the adverse environmental conditions associated with prematurity rather than a result of premature birth per se. Glaser, Parmelee and Plattner (1950) concluded that premature babies grow faster than mature babies during the first eight months of life and rapidly reduce their initial handicap.This list of studies could be expanded many times without yielding any more consistent guide to the physical prognosis of prematurity. There appear to be two reasons for this. First, in all studies a relatively large proportion of surviving premature children could not be traced. For example, Capper, even though he enlisted the help of the Viennese police, lost 54% of the children he was looking for, Hess et al. lost 75 % and Drillien 60 %. These great losses would not matter if it were certain that a representative sample of the premature children had been traced. But