High blood sugar levels in the morning in juvenile type 1 diabetics may be caused by a Somogyi phenomenon (counter-regulation after nocturnal hypoglycaemia) or insulin resistance in the morning hours (dawn phenomenon). To enable differentiation between the two, 1,562 blood sugar profiles (24 h, 3 h, 6 h) were determined in 161 children and juveniles (74 boys, 87 girls; mean age 10.8 [1.0-19.7] years) with type 1 diabetes mellitus. In accordance with the mechanism of the dawn phenomenon there was a close positive correlation between the blood sugar levels in the night and morning (r = +0.696; P less than 0.0001); the mean fasting blood sugar level was about 60 mg/dl above the 3 h value. Low nocturnal blood sugar levels as a possible cause of a high morning blood sugar (greater than 250 mg/dl) was demonstrated in fewer than 1% of profiles. On the other hand, the probability of nocturnal hypoglycaemia rose exponentially in the presence of low morning fasting blood sugar levels. Thus, if the morning level was below 80 mg/dl, the blood sugar levels at 3 h was below 50 mg/dl in 41.2%. This indicates that high morning blood sugar levels result from the dawn phenomenon and require a higher evening dose of slow-release insulin. But if the morning blood sugar values are clearly below 100 mg/dl, the cause may be nocturnal low blood sugar levels and the evening insulin dose should, therefore, be reduced.