A method and device are provided for self-monitoring a patient's blood glucose condition to determine a long-term effect of the patient's behavior on his blood glucose level. A sequence of measurements of the blood glucose level is applied during a day, the sequential measurements are repeated during a predetermined time period from several days to several weeks, and measured data are collected. The measured data is analyzed to determine a distribution of the average glucose values within said predetermined time period, thereby providing a feedback for the patient or an authorized person.