In 27 acute stroke patients with hemi-motor deficit blood gases (paO2, paCO2 and blood pH) were determined within 72 hours and related to level of consciousness, site of brain lesion, findings of haemorrhage into the cerebral spinal fluid, extent of motor impairment and concomitant medical disorders. Sixteen subjects were followed for three weeks with repeated blood-gas sampling and assessment of motor control. Respiratory alkalosis occurred in 37%, hypoxia in 7% but acidosis in none. Blood gas abnormalities were significantly and positively associated only with the extent of motor impairment. Only small changes in blood gases were found during the three weeks follow-up. Initial findings of respiratory alkalosis predicted poor motor recovery during the follow-up period.