AIMS: To investigate whether increased risk of severe hypoglycaemia in early pregnancy is related to pregnancy-induced changes in renin-angiotensin system (RAS) activity in women with type 1 diabetes (T1DM). METHODS: Severe hypoglycaemic events the year preceding pregnancy were recorded retrospectively in 107 consecutive pregnant women with T1DM. Events during pregnancy were recorded prospectively. Measurements of ACE, renin and angiotensinogen were determined at 8, 14, 21, 27 and 33 weeks and postpartum. RESULTS: The rate of severe hypoglycaemia was 1.1 and 5.3 events/patient-year the year preceding pregnancy and in first trimester, respectively (p<0.0001). Levels of ACE, renin or angiotensinogen did not differ between women with and without severe hypoglycaemia during pregnancy. Multivariate regression analysis identified a positive association between rate of severe hypoglycaemia the year preceding pregnancy and postpartum ACE activity (relative rate of severe hypoglycaemia above versus below median ACE activity: 4.4 (CI: 1.7-11.9), p=0.003). No association was found between severe hypoglycaemia during pregnancy and renin angiotensin system activity at 8 weeks. CONCLUSIONS: In early pregnancy increased RAS activity does not explain the 5-fold increase in severe hypoglycaemia in women with T1DM. A positive association between occurrence of severe hypoglycaemia and ACE activity outside pregnancy was demonstrated.