Aims/hypothesis. To investigate the association between glycaemic control and hypertensive pregnancy complications. Methods. From 1988 to 1997, we followed up 683 consecutive non-selected pregnancies in women with Type I (insulin-dependent) diabetes mellitus. Glycaemic control was assessed by assay of HbA_{1 c}. Pre-eclampsia was defined as diastolic blood pressure of 90 mmHg or more at the end of pregnancy after an increase of 15 mmHg or more, combined with proteinuria of 0.3 g or more for 24 h. Pregnancy-induced hypertension was defined similarly but without proteinuria. The same criteria were applied to a control group of 854 non-selected non-diabetic women. Results. Pre-eclampsia developed in 12.8 % of the women with diabetes (excluding those with nephropathy before pregnancy) and in 2.7 % of the control women (odds ratio 5.2; 95 % CI 3.3–8.4). In multiple logistic regression, glycaemic control, nulliparity, retinopathy and duration of diabetes emerged as statistically significant independent predictors of pre-eclampsia. The adjusted odds ratios for pre-eclampsia were 1.6 (95 % CI 1.3–2.0) for each 1 % increment in the HbA_{1 c} value at 4–14 (median 7) weeks of gestation and 0.6 (0.5–0.8) for each 1 % decrement achieved during the first half of pregnancy. Changes in glycaemic control during the second half of pregnancy did not significantly alter the risk of pre-eclampsia. Unlike pre-eclampsia, the risk of pregnancy-induced hypertension was not associated with glycaemic control. Conclusion/interpretation. In women with Type I diabetes, poor glycaemic control is associated with an increased risk of pre-eclampsia but not with a risk of pregnancy-induced hypertension. [Diabetologia (2000) 43: 1534–1539]