From Jan. 1987-Jun. 1993, 37 cases of insulin dependent diabetes mellitus (IDDM) pregnant women and 10 gestational diabetes mellitus (GDM) mothers requiring regular insulin (RI) treatment during pregnancy were recruited. A comparative study of the daily RI requirement was carried out in women breast-feeding (BF) or not (non-BF) at different periods: before pregnancy, during gestation, postpartum and at present. Results showed no difference of RI requirement between BF and non-BF groups before pregnancy, during gestation and currently, but a significant decrease of RI requirement (P < 0.05) among BF mothers in postpartum period. Of the 10 GDM cases, 3 of them who did not breast feed their babies required further RI for 4-7 days in the postpartum period, whereas 7 BF mothers did not need any RI after delivery with blood glucose levels remaining within normal range. Based on the above analysis one may conclude that BF can reduce the RI requirement of IDDM and GDM mothers in the postpartum period. It is thought that more energy is needed in the process of producing milk, and serum glucose is the main substance for lactose synthesis, thus blood glucose level of BF mother is decreased, and so is the RI requirement. Therefore, mothers with diabetes mellitus are encouraged to breast feed their babies.