Of 3841 children born alive in Malmö in Sweden in 1966, 188, 4.9% had a birth weight of larger than 2500 g. We studied these children with their families and a control in order to form an opinion on the role played by certain social and biological factors in the aetiology of LBW in a country with a relatively high average standard of living. Biological factors were analysed, such as the mother's age, parity, stature, and weight, as well as purely socio-economic factors such as the mother's civil staatus, the social group of the mother and father, income, mother's allowance in case of illness, social help or investigations regarding one or more members of the family, as well as the frequency of immigrants. The combined effect of various socio-economic factors was judged by help of a "social score". The results were analysed for 3 different groups of low birth weight, appropriate for gestational age, small for gestational age and multiple births. Judging from our investigation, biological factors probably play the greatest role in the question of LBW but these factors are difficult to distinguish from socio-economic factors which probably still play a certain role in the group preterm children, appropriate for gestational age.