OBJECTIVE: Cerebral vasoreactivity to pharmacologically induced hypercapnia is impaired in poorly controlled patients with type 1 diabetes but otherwise free from microangiopathy. However, whether this response is also compromised during exercise, a daily-life physiological condition challenging regional cerebral hemodynamics, is unknown. We aimed to investigate prefrontal cortex hemodynamics during incremental maximal exercise in patients with uncomplicated type 1 diabetes, taking into account long-term glycemic control as well as exercise- and diabetes-influenced vasoactive stimuli. METHODS: Two groups of patients (type 1 diabetes with adequate glycemic control [T1D-A], n = 8, HbA1c 6.8 ± 0.7% [51 ± 7.7 mmol/mol]; type 1 diabetes with inadequate glycemic control [T1D-I], n = 10, HbA1c 9.0 ± 0.7% [75 ± 7.7 mmol/mol]) were compared with 18 healthy control subjects (CON-A and CON-I) matched for physical activity and body composition. Throughout exercise, near-infrared spectroscopy allowed investigation of changes in oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb), and total hemoglobin (THb) in the prefrontal cortex. Venous and arterialized capillary blood was sampled during exercise to assess for factors that may alter prefrontal cortex hemodynamics and oxygenation. RESULTS: No differences were observed between T1D-A and CON-A, but VO2max was impaired (P < 0.05) and cerebral blood volume (THb) increase blunted (P < 0.05) in T1D-I compared with CON-I. Nonetheless, O2Hb appeared unaltered in T1D-I probably partly due to blunting of simultaneous neuronal oxygen extraction (i.e., a lower HHb increase; P < 0.05). There were no intergroup differences in arterial oxygen content, Paco2, pH, [K(+)], and free insulin levels. CONCLUSIONS: Maximal exercise highlights subtle disorders of both hemodynamics and neuronal oxygenation in the prefrontal cortex of poorly controlled patients with type 1 diabetes. These findings may warn clinicians of brain endothelial dysfunction occurring even before overt microangiopathy during exercise.