Gestational diabetes mellitus (GDM) is associated with adverse foetal and maternal outcomes, and identifies women at risk of future Type 2 Diabetes Mellitus (T2DM). Breast-feeding may improve postpartum maternal glucose tolerance. We prospectively examined the prevalence of postpartum dysglycaemia after GDM and examined the effect of lactation on postpartum glucose tolerance. We compared postpartum 75g oral glucose tolerance test (OGTT) results from 300 women with GDM and 220 controls with normal gestational glucose tolerance (NGT). Breast-feeding data was collected at time of OGTT. Postpartum OGTT results were classified as normal [fasting plasma glucose (FPG) < 5.6mmol/l, 2-h < 7.8 mmol/l] and abnormal [impaired fasting glucose (IFG), FPG 5.6-6.9 mmol/l; impaired glucose tolerance (IGT), 2-h glucose 7.8-11 mmol/l; IFG+IGT; T2DM, FPG > or = 7 mmol/l +/- 2h glucose > or = 11.1 mmol/l]. 6 (2.7%) with NGT in pregnancy had postpartum dysglycaemia compared to 57 (19%) with GDM in index pregnancy (p < 0.001). Non-European ethnicity (OR 3.40, 95% CI 1.45-8.02, p = 0.005), family history of T2DM (OR 2.14, 95% CI 1.06-4.32, p = 0.034) and gestational insulin use (OR 2.62, 95% CI 1.17-5.87 p = 0.019) were associated with persistent dysglycaemia. The prevalence of persistent hyperglycaemia was significantly lower in women who breast-fed versus bottle-fed postpartum (8.2% v 18.4%, p < 0.001). Breast-feeding may confer beneficial metabolic effects after GDM and should be encouraged.