Aims/hypothesis. To evaluate the value of 24-h blood pressure monitoring compared to office blood pressure and urinary albumin excretion in predicting pre-eclampsia in Type I (insulin-dependent) diabetes mellitus.¶Methods. The study included 136 consecutive pregnancies in Caucasian women with Type I diabetes before gestation without diabetic nephropathy, anamnestic hypertension or early abortion. Values of urinary albumin excretion and office blood pressure before pregnancy and the HbA_1C value at the time of conception were obtained. Microalbuminuria was defined as urinary albumin excretion of 30–300 mg/24 h in two out of three consecutive urine samples. Single measurements of 24-h urinary albumin excretion, office blood pressure and HbA_1C were done five 5 times during pregnancy. In a subgroup of 74 women 24-h blood pressure measurements were done at 10 and 28 weeks of gestation. Pre-eclampsia was defined as office blood pressure higher than 140/90 mm Hg accompanied by proteinuria above 0.3 g/24 h later than 20 weeks of gestation.¶Results. Urinary albumin excretion and systolic blood pressure were higher before and throughout pregnancy in 14 women developing pre-eclampsia compared with women remaining normotensive (p < 0.001; p < 0.05, respectively). By logistic regression analysis the best predictor for pre-eclampsia was microalbuminuria before pregnancy (p < 0.05) with no additive predictive effect of 24-h blood pressure or office blood pressure measurement. The night:day ratio of blood pressure was similar in the two groups.¶Conclusion/interpretation. Microalbuminuria before pregnancy is the strongest predictor of pre-eclampsia in Type I diabetes. Measuring 24-h blood pressure early in pregnancy did not improve the ability to identify women at risk. [Diabetologia (2000) 43: 927–931]