Regional cerebral blood flow (rCBF) asymmetries were studied in 189 subjects (96 males and 93 females) at rest with the 133Xenon inhalation method using a fixed detector system. rCBF asymmetries in the resting condition were very small, nevertheless a significant (p less than 0.001) effect for their topographical distribution was present, reflecting higher rCBF in the right fronto-temporal and left parieto-occipital regions. rCBF asymmetries were not correlated with age, and there were no significant differences between males and females. Asymmetries are therefore useful from a statistical point of view in detecting rCBF abnormalities in the resting condition: they are more stable than absolute values in normal subjects and no matching according to age or sex is required when statistical comparisons are performed.