The correlation between anterior cruciate ligament (ACL) collagen fibril diameter and aging were studied in subjects aged 15 to 87 years. The samples processed for light and electron microscopy showed statistically significant differences in collagen fibril diameter among young (< 20 years), adult (20-60) and elderly subjects (> 60 years). In the young, the ACL was made up of collagen fibrils which were highly variable in size (range 20-180 nm); the diameter distribution curve was very asymmetrical (mean asymmetry +0.895). In adults and elderly subjects, the maximum diameter had decreased remarkably (120 and 110 nm, respectively) and the diameter distribution curve had become less asymmetrical (mean asymmetry +0.527 and +0.297 respectively). Fibril concentration increased considerably from young (68 fib/mu 2) and elderly subjects (140 fib/mu 2). This reduction in diameter and the relative change in collagen fibril concentration may be related to changes in elastic stiffness. The increase in small collagen fibrils and the marked rise in their concentration may make the ligament more pliable. These findings are similar to those we obtained in Achilles tendon. They demonstrate that both ACL and Achilles tendon, a tissue which responds to unidirectional mechanical forces more than ACL does, show a reduction in diameter value during ageing. These data further suggest that collagen fibril diameter is related to the aging process.