The purpose of this study is to disclose the differences of risk factors between pre-term low birth weight (LBW) infants and full-term LBW infants. The cases studied were all singleton, live, LBW infants born in 2 districts of a certain city from 1986 to 1987. Each case was matched with 2 controls by district, sex and birthday, and a questionnaire concerning the factors related to birth weight was sent to both cases and controls. The questionnaires were answered by 308 (76.6%) cases and 644 (80.1%) controls. The cases and controls who responded were matched again, and the case group was divided into a pre-term group and a full-term group. And each case group was compared with the same control group. The results were as follows. 1) Factors related to a higher incidence of pre-term LBW infants were different from those of full-term LBW infants. 2) The risk factors for pre-term LBW infants were male sex; increased parental age, parental short height and low academic career; maternal smoking, anxiety and night working during pregnancy; and toxemia. 3) The risk factors for full-term LBW infants were female sex; parental short height, light weight and smoking; maternal young age and work during pregnancy; toxemia and hypertension. 4) Discriminant analysis disclosed that toxemia, paternal short height and maternal night work during pregnancy were strongly related to a higher incidence of pre-term LBW infants, and that maternal and paternal light weight, maternal smoking were the more important factors related to a higher incidence of full-term LBW infants.