OBJECTIVE - Repeated severe hypoglycemia has been reported to reduce long-term spatial memory in children with type 1 diabetes. Early exposure to hypoglycemia may be more damaging to cognitive function than later exposure. Our goal was to determine whether the age at which severe hypoglycemia occurs modulates the impact of severe hypoglycemia frequency on long-term spatial memory. RESEARCH DESIGN AND METHODS - We combined data from three independent studies to obtain a sample of children aged 6-18 years with type 1 diabetes (n = 103) and nondiabetic control subjects (n = 60). Each study evaluated previous severe hypoglycemia and tested short (5 s)- and long (60 s)-delay spatial memory with the spatial delayed response task. Type 1 diabetic participants were categorized as having zero, one to two, or three or more severe hypoglycemic episodes and as having their first severe hypoglycemic episode before or after 5 years of age. Information on chronic hyperglycemic (HbA1c values) was also collected. RESULTS - We found that repeated severe hypoglycemia (more than three episodes) reduced long-delay spatial delayed response performance, particularly when severe hypoglycemic episodes began before the age of 5 years. Age of type 1 diabetes onset and estimates of chronic hyperglycemia did not influence performance. CONCLUSIONS - High frequency of and early exposure to severe hypoglycemia during development negatively affects spatial long-term memory performance.