Introduction: This study presents the results of a systematic examination of how parents and patients perceive the effects of acute hyperglycaemia on mood and on intellectual and fine-motor performance.Methods: A random sample of parents and children with type 1 diabetes mellitus (n = 380) were asked via questionnaire about their perception of the effects of hyperglycaemia.Results: Answers were received from 368 children and parents (97%); mean age was 12.5 years (range 3.7 to 19.8 years). Blood glucose levels between 15-18 mmol/l were reported by 68% to affect thinking performance, by 75% to affect mood and emotions and by 53% to affect coordination. The symptoms most commonly associated with hyperglycaemia were (in decreasing order): irritable (64%), short-tempered (60%), moody (56%) unreasonable (43%), aggressive (37%), hyperactive (31%), sad/depressed (27%) confused (23%), clumsy.(18%). Homework was thought to be affected by hyperglycaemia in 36%, sport in 25%, play activity in 32%. Higher mean HbA(lc) since diagnosis was associated with higher reporting of effects on feeling/mood (p < 0.001). Parents were especially concerned about the effects of hyperglycaemia on school and social interaction.Conclusions: The effects of hyperglycaemia on immediate performance are of concern to children and parents. Hyperglycaemia (15-18 mmol/l) is reported to influence emotion and behaviour more than intellectual and fine-motor performance. This may contribute to the adverse psychological consequences of type 1 diabetes mellitus in children.