1.Question: Patients with Parkinson disease (PD) report difficulties walking while performing another task. Dual-task (DT) walking may therefore increase the risk of falls. To date, the effect of DT walking on brain activity is not well understood. The objective of this pilot study was to examine dorsolateral prefrontal cortex activity (DLPFC) during single-task (ST) and DT walking conditions in PD patients. 2.Methods: Thirteen patients with PD (mean age: 66 ± 7 years, 8 men, UPDRS motor score (/108): 23 ± 11, Montreal Cognitive Assessment (MoCA) score: 27 ± 2) and 13 healthy older adults (mean age: 64 ± 7 years, 8 men, MoCA score: 28 ± 3) performed three walking tasks: [1] ST walking, [2] DT walking while counting forward (+1) and [3] DT walking while subtracting (-7). Cortical activity in DLPFC, assessed by changes in oxy-haemoglobin (?HbO2) was measured using functional near infrared spectroscopy (NIRSport). Gait performance (speed, cadence and stride length) was recorded simultaneously using wearables sensors (Gait-Up sensors). 3. Results: In ST walking and DT walking while subtracting conditions, ?HbO2 was higher in patients than in controls (p < 0.05). No significant differences in ?HbO2 were found between ST and the two DT-walking conditions in patients whereas ?HbO2 was significantly higher in DT-walking conditions than in ST walking in controls. As expected, patients had reduced walking speed, cadence and stride length compared to controls, regardless of conditions (p < 0.05). Variability of their cadence increased as the walking task difficulty increased, which is not the case for controls. 4. Conclusion This pilot study suggests that dorsolateral prefrontal cortex plays a major role already during ST walking condition in patients in relatively early stages of PD. This may suggest that patients recruit additional cognitive resources, as a measure of cognitive compensation to perform the ST walking. These findings may have implications for rehabilitation of gait in patients with PD, improving automaticity in simple tasks. Another ongoing study will show the effect of an exercise-based intervention program (SIROCCO program) on DLPFC activity while walking in the same group of patients with PD.