Introduction and Objectives Patients with idiopathic pulmonary fibrosis (IPF) present respiratory derangements at rest and during exercise, accompanied by exercise intolerance. Some patients may develop profound exertional desaturation even without resting hypoxemia. Evidence suggests the involvement of reduced cerebral-oxygenation in exercise intolerance. We aimed to examine (i) differences in cerebral-oxygenation during exercise between IPF patients with and without isolated exertional desaturation, (ii) whether the impairments in cerebral-oxygenation are detected at similar exercise intensity, and (iii) correlations between cerebral-oxygenation indices, disease severity, and 6-min walk test (6MWT). Materials and Methods Patients with IPF (n = 24; 62.1 ± 9.3 years) without resting hypoxemia underwent cardiopulmonary exercise testing (CPET) with cerebral-oxygenation monitoring via near-infrared-spectroscopy (NIRS). Βased on their pulse-oxymetry saturation (SpO2) during CPET, patients were divided into the “exertional-desaturators” group (SpO2nadir≤89% and ≥6% drop in SpO2) and the “non-exertional-desaturators” group (SpO2nadir≥90% and ≤5% drop). Results During CPET, the “exertional-desaturators” group exhibited lower oxygenated-hemoglobin (-0.67 ± 1.48 vs. 0.69 ± 1.75 μmol/l; p textless 0.05) and higher deoxygenated-hemoglobin (1.67 ± 1.13 vs. 0.17 ± 0.62 μmol/l; p textless 0.001) than the “non-exertional-desaturators” group. A different pattern (p textless 0.01) in cerebral-oxygenation responses was observed in the two groups. In exertional-desaturators oxygenated-hemoglobin declined below baseline even at low/moderate-intensity exercise (p textless 0.05), whereas, in non-exertional-desaturators cerebral-oxygenation declined (p textless 0.05) at high-intensity exercise. Cerebral-NIRS indices correlated (p textless 0.05) with CPET-duration, dyspnea, diffusion capacity, and 6MWT. Conclusions During incremental exercise, patients with IPF and exertional desaturation present a significant decline in cerebral-oxygenation even during low-intensity exercise. Our findings support the implementation of longer-duration rehabilitation programs in IPF so that lower intensity exercise can be applied at the initial stages. (NCT 03683082)