No studies have previously compared ultrastructural differences in human posterior cruciate ligaments in different age groups, but such differences may be of importance in the reconstruction of this ligament. We used light and transmission electron microscopy to compare ultrastructural differences in the posterior cruciate ligaments of 36 patients by age and sex. We found no differences between the sexes or in the groups aged between 30 and 59 years. However, collagen fibers with variable diameters were observed in those aged 10-19 years, with many oxytalan fibers between the collagen fibers. We also found a decrease in collagen fiber diameter and increase in collagen fibril concentration with aging, with a maximum increase in collagen fibril concentration in those aged 60-69 years.