The width of the articular space of the lateral atlantoaxial joint was measured to evaluate normal range values and possible variations according to age and sex. The study was based on X-rays of the lateral atlantoaxial joint of 52 females (age range: 18-82 ys) and 50 males (age range: 20-82 ys). The radiograms were taken using a semiautomatic computerized morphometric system (Videoplan II, Image Analysis system-Kontron, Munich, Germany). The articular space width of the lateral atlantoaxial joint was measured on both sides, at the medial, lateral and median levels. The mean value at these levels, on both sides, was calculated. Regression analysis was performed to evaluate the relationships between the width of the articular space at the three different levels and the patient's age and gender. No significant differences were found in the mean values of the articular space width at the three levels as regards gender. However, a significant inverse correlation was found between the articular space width and the patients' age in both males and females. Comparative analysis of regression lines showed that the slope of these lines was not significantly different when the sexes were compared. On the contrary, the intercept of the regression lines at the medial and median levels was significantly higher in the females than in the males; the contrary was encountered at the lateral level. This feature is probably due to the different types of inclination of the articular facets of the axis or of those of the atlas in the two sexes.