Objective. Invasively measured arterial blood pressure (ABP) is associated with complications, while non-invasively measured ABP is generally considered risk-free. This study aimed to investigate the reliability of non-invasive ABP measured using finger-cuff volume-clamp device compared to invasive ABP measured by an arterial catheter in patients with aneurysmal subarachnoid haemorrhage (SAH).Approach. In 30 patients admitted for neurointensive care with SAH, invasive and non-invasive ABP were recorded simultaneously. Reliability was assessed for mean, diastolic and systolic ABP separately using intraclass correlation coefficient (ICC) agreement for each full period and each 3 s average.Main results.A median of 3 (IQR: 2-3) periods were included for each participant. The full periods (n = 81) showed an ICC of 0.34 (95% CI: 0.14-0.52), 0.31 (95% CI: 0.10-0.49), and 0.20 (95% CI: 0.00-0.39) for mean, diastolic, and systolic ABP, respectively. Three-second averages (n = 33 786) for mean (ICC: 0.35; 95% CI: 0.33-0.36), diastolic (ICC: 0.25; 95% CI: 0.25-0.28), and systolic ABP (ICC: 0.26; 95% CI: 0.18-0.33) yielded similar findings. Pearson's correlation coefficient showed anR2of 0.15 (p < 0.001), 0.15 (p < 0.001), 0.06 (p = 0.027) for mean, diastolic and systolic ABP, respectively.Significance.In patients with SAH, non-invasive measurement of ABP using the widely used Nano system from Finapres Medical Systems-a finger-cuff volume-clamp device (Finapres, Chennai, India) showed poor reliability and therefore cannot be used interchangeably with invasively measured ABP.