Purpose <p>Patients with diffuse lower-grade gliomas (LGG) often suffer from mental fatigue. In healthy subjects, mental fatigue has a negative impact on cognitive functioning. This relation may be more complex in LGG, where tumor localization and growth rate also impact brain function. Our aim was to investigate how self- and observer-reported variables of mental fatigue and cognitive functioning were connected before tumor treatment.</p> Methods <p>Consecutive patients scheduled for surgery due to presumed LGG were screened (<i>n</i> = 157). LGG was presumed if the mass was suggestive of diffuse glioma, but without significant contrast enhancement. Isocitrate dehydrogenase (IDH)-mutated WHO grade 2 or 3 gliomas (the LGG group) were analyzed separately. We included 101 patients in the entire cohort, whereas 71 patients constituted the LGG group. Patient data included: (1) self-reported assessments of mental fatigue and cognitive functioning, (2) neuropsychological test performances, and (3) clinical/demographic characteristics. Spearman's partial correlations were calculated between the variables and visualized in a correlation network.</p> Results <p>Cognitive impairment was self-reported by 50% of the entire cohort and 45% of the LGG group, while observer-evaluated testing showed cognitive impairment in 40% and 34% of the cases respectively. Self-reported assessments showed no correlations (≥ 0.3 or ≤-0.3) with neuropsychological test performances. A consistent correlation was seen between self-reported mental fatigue and self-reported cognitive functioning (entire cohort: rho=-0.66, LGG group: -0.64).</p> Conclusion <p>Our results highlight the complexity of evaluating symptoms of mental fatigue and cognitive functioning even prior to surgery. Self-reports and neuropsychological testing were weakly correlated, hence these should be handled complimentary.</p>