We evaluated arterial oxygen saturation (SaO₂) and heart rate in acute stroke patients to determine whether routine positioning affected these physiological parameters. Measurements were recorded at the bedside non-invasively in five different positions assigned in random order each maintained for 10 min. One hundred and twenty-nine patients examined within a median of 72 h, lying on the left side resulted in slightly lower SaO₂ than lying on the right side, which was statistically significant in the patients with a right (n = 66), but not left, hemiparesis. Patients able to sit in a chair (n = 65), who mostly had less severe strokes, had a significantly higher mean SaO₂ and heart rate when sitting in the chair than when placed in any other position. About 10% of patients, especially those with a severe stroke, with right hemiparesis and concomitant chest disease, experienced falls in SaO₂ to 90% or less for 2 min in certain positions; the hypoxia was more likely when they were lying on their left side. These results may have implications for current practice and for future patient positioning strategies to improve outcome after stroke.