We examined the relationships between the density of the subchondral bone of the proximal end of the tibia and the location and condition of the overlying articular cartilage and menisci. We took coronal sections of the tibial plateaus and of the overlying articular cartilage and menisci from human knee joints that were grossly free of osteoarthritis. The specimens were examined for the presence of cartilage fibrillation using the India-ink technique of Meachim. Bone density was determined using the Quantimet picture-point analyzer. Fibrillation of articular cartilage was observed only in the articular cartilage that was not covered by meniscus and lying mesial to it, which supports the concept that the meniscus confers a protective effect on the articular cartilage. The density of the bone underlying the meniscus was lower, and was inversely related to the thickness of the meniscus and directly related to the thickness of the articular cartilage. In four knees from which the menisci were absent, the bone density was increased at the periphery. We suggest that there is a trinitarian relationship between the meniscus, cartilage, and subchondral bone, in which structural changes in any one of the three causes secondary pathological adaptive changes in the other two.