Owing to difficulties in measuring ventilation symmetry, good evidence of different right/left respiratory movements has not yet been provided. We investigated tidal volume differences between paretic and healthy sides during quiet breathing, voluntary hyperventilation and hypercapnic stimulation in hemiparetic patients. We studied eight hemiparetic patients and nine normal sex- and age-matched subjects. Right and left sided tidal volume was reconstructed using opto-electronic plethysmography. In controls no asymmetry was found in the study conditions. Tidal volumes of paretic and healthy sides were similar during quiet breathing, but paretic tidal volume was lower during voluntary hyperventilation in six patients, and higher during hypercapnic stimulation in eight patients (p=0.02). The ventilatory response to hypercapnic stimulation was higher on the paretic than the healthy side (p=0.012). In conclusion, hemiparetic stroke produces asymmetric ventilation with an increase in CO2 sensitivity and a decrease in voluntary ventilation on the paretic side.