The predominant collagen fiber orientation in newly formed osteons was determined by polarized light microscopy. Osteons were classified into five categories according to the proportion of their cross section formed by dark lamellae. In dark lamellae most fibers run longitudinally or on a step spiral while they follow a circular or shallow spiral course in light lamellae. Types 1 and 2 are 'light' osteons; types 4 and 5 are 'dark', and type 3 is 'intermediate'. A significant correlation was found between the predominant collagen fiber orientation in osteons and the age at which their formation occurs. Significant differences were found also in the collagen fiber orientation in osteons among the different quadrants of the same bone.