We aimed to investigate the effects of age onset of diabetes, glycemic control and frequency of hypoglycemia on neurocognitive functions in type I diabetic children. Sixty type I diabetic children with diagnosis before (Group 1) or after (Group 2) five years of age and 40 healthy children were tested. Wechsler Intelligence Scale for Children Revised (WISC-R), Stroop Test, and Visual Auditory Digit Span Test Form B were applied to all children in the two groups. Neurocognitive functions such as visual perception, short-term memory and selective attention were seen to be negatively affected at a significant level. Group 1 patients with poor glycemic control were found to have significant dysfunction in verbal, performance and general intelligence. Neurocognitive functions were negatively affected by early onset of diagnosis, poor glycemic control and frequent hypoglycemia in children with type I diabetes mellitus. We suggest that negative effects on neurocognitive functions in type I diabetes should be considered in the follow-up of these patients.