Objective: Controversy surrounds whether early onset Type 1 diabetes mellitus (T1DM) may significantly impact cognitive functions. Whereas past research had theorized that severe hypoglycemic episodes might be associated with cognitive dysfunction (Ryan et al., 1985), more recent work has associated hyperglycemic episodes with compromised neurocognitive functions (Patiῆo-Fernández, et al., 2010), including executive dysfunction (Cato et al., 2014). In contrast, one prospective longitudinal study (Northam et al., 2009) observed no significant cognitive differences between children with well-controlled T1DM compared to healthy controls. Methods: Neuropsychological case study of a 22-year old, right-handed, Caucasian male with early onset (i.e. 4 years old) T1DM and documented chronic and severe hyper- and hypo-glycemic episodes that led to diabetes-related seizure events at age 6 and 7 – alongside an otherwise complicated and contributory medical history. Results: Overall WAIS-III summary scores were within the low average range (FSIQ = 88) with remarkably uniform index composites (VCI, POI, WMI and PSI = 88). Within the context of a life history of extreme executive dysfunction, performance on formal assessment measures was mixed (i.e., WCST – intact to superior; D-KEFS Tower Test – borderline achievement with extreme inefficiency]. Conclusion: Research outcomes on the cognitive effects associated with T1DM have been varied. Important factors appear to include early onset of T1DM and the occurrence of extreme fluctuations in blood sugar levels. This case appears to provide affirmative support that extreme glycemic fluctuations early in life may contribute to brain alterations that can result in focal impairments in cognitive functions, particularly in the executive domain.