What factors determine birth weight and do they have any biological significance for the future health and welfare of the individual? These are important questions for all paediatricians and four groups of experts have been approached to independently give their thoughts on key areas. The aim was for concise pithy comments and opinions, so the length of the articles was rigourously controlled. Firstly, are there genetic factors that contribute to birth weight, secondly does maternal nutrition determine size, then are there social influences on birth weight? Is birth weight of biological significance for the individual or the species? Patrick Cartlidge, Senior Lecturer in child health in Cardiff, kindly refereed this symposium. Please feedback your views and comments to the rapid response section of the journal website at www.archdischild.com.