Our objective was to determine scores on tests of neurocognitive functioning in children diagnosed with diabetes before age 10 years and to determine the association of age of diagnosis, duration of diabetes, subtle hypoglycemia, severe hypoglycemia, and history of hypoglycemic seizures with these neurocognitive test scores. Fifty-five of 62 eligible patients with a mean age of 7.9 +/- 1.6 years followed in our center were given the Woodcock-Johnson Psychoeducational Battery, Beery Developmental Test of Visual-Motor Integration, Finger Tapping, Grooved Pegboard, and Verbal Selective Reminding tests to evaluate the following domains: memory/attention, visual-perceptual, broad cognitive function, academic achievement, and fine motor speed/coordination. Fifteen age-matched siblings served as controls. Twenty-seven subjects were less than 5 years of age when diagnosed with diabetes, the mean age at diagnosis was 4.5 +/- 2.1 years of age, and mean diabetes duration was 2.6 +/- 2.0 years. Eighteen patients had a history of severe hypoglycemia, eight of whom had hypoglycemic seizures. The mean HbA1c was 7.8 +/- 1.1% for the year prior to testing. Our results showed that the overall mean scores for the extensive neurocognitive battery were within the normal range and were comparable to the scores of the age-matched sibling controls. Age of diagnosis and duration of diabetes did not relate to neurocognitive test results. Mean HbA1c had a negative association with some tests of memory/attention (p < 0.03-0.04) and academic achievement (p < 0.005-0.03), while number of blood glucose levels less than mg/dL had a positive association with memory/attention (p < 0.004-0.04), verbal comprehension (p < 0.03) and academic achievement (p < 0.018-0.05). There was no association of neurocognitive test scores with severe hypoglycemia, but subjects with history of hypoglycemic seizures had a decrease in scores on tests assessing memory skills (p < 0.03) including short term memory and memory for words. These data suggest that overall neurocognitive test scores were within the normal range and comparable to controls. However, specific aspects of neurocognitive functioning may be adversely affected by having had a hypoglycemic seizure, but not by episodes of severe hypoglycemia without seizure. Lower HbA1c and an increase in the number of blood glucose levels less than 70 mg/ dL (subtle hypoglycemia) which were associated with higher scores in some domains of academic achievement and memory suggests that stable glycemia may influence cognitive abilities and/or that successful diabetes management requires cognitive skills. Strategies to diminish the risk of seizures with hypoglycemia should be investigated.