The effect of voluntary hyperventilation was assessed in 22 cluster headache patients (8 in a cluster period and14 in a remission) and 19 healthy individuals. Using an ear oximeter and a capnograph with a nasal probe, the oxygen saturation (SaO2) and the end-tidal CO2 were monitored continuously. During the hyperventilation per se, cluster headache patients and controls showed absolute values of end-tidal CO2 and of SaO2 Of the Same order of magnitude. In the posthyperventilation phase, however, the average of the lowest SaO2 levels was lower in controls than in cluster headache patients. In the posthyperventilation phase, headache patients outside the cluster period showed a trend more similar to that of the controls with respect to SaO2 than did those inside the cluster period. The observed discrepancy might, if reproducible, be a consequence of an altered chemoreceptor sensitivity in cluster headache patients during the bout.