Background. Frailty is a multidimensional geriatric syndrome characterized by a decline across multiple physiological systems [1]. It is accompanied by an increased vulnerability to stressors and is associated with adverse postoperative outcomes [2]. Frailty is linked to poor cognitive function and dementia, as well as structural changes of the brain [3,4]. However, it has not been investigated whether frail patients present with alterations in the electroencephalogram (EEG). We compared pre- and intraoperative EEG parameters in frail and non-frail patients. Methods. Patients aged ≥65 years undergoing elective surgery were included in this prospective observational study (NCT02265263; EA2/092/14). Frailty was assessed one day before surgery using a modified version of the Fried frailty phenotype comprising five criteria: slowness, weakness, exhaustion, weight loss and a level of low activity. Continuous frontal EEGs were recorded and analysed during 2 min periods before induction of anesthesia (preoperative) and during a stable intraoperative period (intraoperative). Only patients who received propofol for induction and volatile anesthetics for maintenance of anesthesia were included in the analyses. We compared the power spectrum within different frequency bands (δ: 1.5-4Hz, θ: 4-8Hz, α: 8-12Hz; β: 12-30Hz), as well as the spectral edge frequency (SEF) of frail and non-frail patients. Results. Of the 112 patients included in this study 14 (12,5%) were frail. No significant difference in power or SEF was found during the preoperative EEG measurements between the two groups. In the intraoperative EEG, frail patients presented with significantly lower a-power compared to non-frail patients (a-power: non-frail −3.67 [−7.77; −1.08] dB vs frail −8.71 [-13.75; −7.11] dB, p = 0.006). Moreover, intraoperative SEF values were significantly lower in frail patients (SEF: frail 10.71 [7.96; 11.34] Hz vs non-frail 13.70 [11.36; 15.62] Hz, p=0,001). In a multiple linear regression model adjusting for age, sex, ASA score, GDS Score, dose of anesthetics and cognitive abilities, frailty remained significantly associated with lower SEF values ( p = 0,011). Conclusion. Preoperative frailty was associated with lower intraoperative a-power, as well as lower SEF values. This should be taken into consideration when interpreting intraoperative EEGs of elderly patients.