In a search for predictors of nephropathy development, albumin excretion rate (AER), ambulatory blood pressure, and parental hypertension were assessed in 40 type 1 diabetic patients and 27 normal siblings (age<18 years) during a 2-year follow-up period. A double-antibody kit and an automated device were used for measuring 24-h AER and ambulatory blood pressure monitoring (ABPM), respectively. Patients had higher 24-h and daytime diastolic blood pressure (DBP), diastolic load, and daytime heart rate than siblings. Patients with hypertensive parents had higher 24-h DBP and diastolic load than patients with normotensive parents and all siblings. Non-dipping was more frequent in children with hypertensive parents ( P<0.05). Both diabetes ( P<0.001) and parental hypertension ( P<0.05) had independent effects on longitudinal AER (average AER during follow-up). Patients with intermittent or persistent microalbuminuria showed a trend towards higher diastolic load ( P<0.05); the latter group had higher 24-h DBP ( P<0.01). Longitudinal AER correlated with 24-h DBP ( P<0.01) and maternal mean blood pressure ( P<0.05). Since changes in blood pressure preceded persistent microalbuminuria, ABPM might help to identify diabetic children prone to nephropathy.