Context Cases of ischaemic brain damage have been reported in relatively healthy patients undergoing shoulder surgery in the beach chair position. Unrecognised cerebral hypoperfusion may have contributed to these catastrophic events, indicating that routine anaesthesia monitoring may not suffice. Near-infrared spectroscopy (NIRS) provides a non-invasive, continuous method to measure regional cerebral oxygen saturation (rS(c)O(2)). Objectives The aim of this clinical investigation was to evaluate the prevalence of regional cerebral oxygen desaturation in patients undergoing shoulder surgery in the upright position during routine anaesthesia management. We also aimed to identify some causal factors for cerebral desaturation. Design Prospective, observational, blinded study. Setting University hospital. Observation period from 19 05 2008 to 26 08 2008. Patients Twenty consecutive adult patients presenting for elective shoulder surgery under general anaesthesia in the beach chair position were enrolled. Patients with clinically apparent neurological or cognitive dysfunction were excluded. Interventions Routine anaesthesia management and standard monitoring were used. The responsible anaesthesiologist was blinded to the rS(c)O(2) data and was not informed about the purpose of the study. Main outcome measures The prevalence of cerebral oxygen desaturation was measured. Results With beach chair positioning, rS(c)O(2) decreased significantly from 79 +/- 9 to 57 +/- 9% on the left side and from 77 perpendicular to 10 to 59 perpendicular to 10% on the right side (P<0.001). A relative decrease in rS(c)O(2) of more than 20% occurred in 80% of patients when the beach chair position was adopted. Postural decreases in cerebral oxygenation were related to blood pressure (r = 0.60, P = 0.007) and end-tidal carbon dioxide concentration (r = 0.47, P = 0.035). Conclusion The high prevalence of significant cerebral oxygen desaturation during shoulder surgery in the upright position underlines the need for close monitoring. NIRS might constitute a valuable technique to detect cerebral hypoperfusion in this high-risk group of patients.