One of the most frequent and important complications of IDDM is hypertension. It begins usually in adulthood and is rare in children. In order to study the behaviour and control of BP in IDDM children and adolescents we analyzed the BP levels of 106 patients (48 males, 58 females; age 1.5-16 yrs) in relation to sex, age, duration of the disease, and different parameters of metabolic control; moreover we studied the modifications of BP levels with years (tracking). BP levels, registered every 3-6 mos, were compared to the standard levels for age of the local population (2000 students between 7 and 16 yrs of age) and expressed as standard deviation scores (SDS) of the means. For each subject a line describing the change of the SDS over time was calculated by the method of least squares: the slope of this line is called trend and represents the tendency of the BP to increase or maintain stable or decrease with time, i.e to develop or not hypertension. All patients, except one 16 y. old girl, had normal BP and no microalbuminuria, but 10 of them presented with mean levels in the upper quartile and a constantly upward BP trend. Two of these patients showed after a 2 year follow-up stable hypertension and microalbuminuria. Moreover, an analytical and statistical study pointed out that BP levels of IDDM children seem to be influenced in addition to age, sex, height, weight, ponderal excess, as the general population, by the duration of the disease the insulin dose and some metabolic parameters (HbA1, HbA1c, glycemia, creatininemia).(ABSTRACT TRUNCATED AT 250 WORDS)