Cerebral blood flow (CBF) was measured twice with the 133Xenon inhalation technique, under resting conditions and during 5% CO2 inhalation, in 13 patients with generalized anxiety disorder and a group of normal volunteers of comparable age and sex distribution. CO2 inhalation was associated with similar increases in end-tidal CO2 (PECO2) and CBF. Neither group showed statistically significant increases in state anxiety. However, when subjects (both patients and controls) who became anxious during CO2 inhalation were compared with those who did not, on associated CBF changes, significant differences emerged. The former showed less marked CBF increase as compared to the latter in the absence of any significant differences between the two groups on PECO2 during the second measurement. Changes in state anxiety and CBF showed a statistically significant inverse correlation for the entire group.