Acute psychological stress may play a role in the glycaemic instability of some patients with type I diabetes through an increased secretion of insulin-counteracting hormones. To examine the validity of this hypothesis, we subjected to a video-recorded public-speaking stress seven healthy persons, six type I diabetics with stable blood glucose levels and six type I diabetics with unstable or brittle diabetes (with more than 10 hypoglycaemia/month and frequent hyperglycaemia). During the test and on a control day, heart rate, blood pressure, plasma ACTH, cortisol, catecholamines and prolactin were measured. The comparison between the stable and unstable diabetics during the stress session by two-way analysis of variance (group/time) showed a significant difference for heart rate, blood pressure, ACTH and cortisol. Psychological interview showed that most unstable diabetics perceived a link between life stress and their blood glucose control. The unstable patients had much more difficulty in verbalizing their emotions. Our study shows that the two groups of diabetic patients display distinct cardiovascular and neuroendocrine responses to psychological stress, as well as distinct psychological profiles. In conclusion, hormonal response to an acute psychological stress is more pronounced in brittle diabetes and might be one of its pathogenic factors.