This study investigated how hypoglycaemia affects mood, and whether psychological characteristics mediate an emotional disturbance during hypoglycaemia. Ten IDDM patients (1 female) were studied, aged [mean (range)] 28 (20-37) years, body mass index (BMI) 23.4 (19.6-25.7) kg/m2 diabetes duration 11 (7-15) years, HbA1c 7.5 (6.0-8.7)%. Patients underwent a standardized stepwise, hypoglycaemic, hyperinsulinaemic clamp, performed in steps of 0.5 mmol/l from 4.0 mmol/1 to a glucose nadir of 2.0 mmol/l. At euglycaemia, the Symptom Checklist-90 (SCL-90) was administered. The Profile of Mood States (POMS) was completed at 4.0 and 2.5 mmol/l, while the feelings chart of the COOP/WONCA was filled out at euglycaemia and all hypoglycaemic steps. Results: The t-test for the POMS demonstrated a significant increase for anger (P < 0.05). COOP/WONCA scores displayed a progressive negative change in mood for each hypoglycaemic step, albeit with a large interindividual variability. Hostility (SCLr90) significantly interacted with anger (POMS) at 2.5 mmol/l (P < 0.05). Conclusions: Progression of hypoglycaemia negatively alters the overall mood state. IDDM patients characterised by high scores on hostility (SCL-90) apparently are more prone to experience an increase in anger (POMS) during hypoglycaemia.