Regional cerebral blood flow was measured by the 133-xenon inhalation method during resting in 38 healthy men and 38 healthy women matched pairwise for age in the range 18-72 years. The results showed 11% higher global flow level in the women in all ages. A similar and significant regression of flow by age was seen for both sexes. The regional flow distribution also showed some sex-related differences. Frontal regions showed an asymmetry in the men with higher values on the right side. The female flows were more symmetric. As a hypothesis, it is suggested that the higher flow level in women may be a systemic phenomenon. In fact, other authors have found a higher cardiac index in females. The sex differences in regional flow pattern might be due to differences in the functional organization of the cortex and/or to differences in the mental processes of the "resting" state.