Early-onset insulin-dependent diabetes mellitus (IDDM) is linked to subsequent learning deficits. To investigate the relationship of learning deficits to metabolic control, 23 children with IDDM (age at testing 71 ± 21 mo, age at diagnosis 35 ± 15 mo) diagnosed before 5 yr of age were followed for periods of 6–78 mo. Mean glycosylated hemoglobin (HbA,), episodes of severe hypoglycemia, and frequency of self-monitoring blood glucose (SMBG) measurements r = –.39, P = .037). This relationship was primarily accounted for by the relationship of asymptomatic hypoglycemia to performance on the copying subscale (r = –.42, P = .022). Children with frequent asymptomatic hypoglycemic episodes had lower mean copying scores and abstract reasoning scores than those with infrequent episodes. These findings are consistent with those of previous crosssectional studies of early-onset IDDM and extend previous results by suggesting that hypoglycemia, even if mild or asymptomatic, may be related to the neuropsychological changes previously described.