Horizontal splits at the interface between the uncalcified and the calcified layers of the articular cartilage have been found in 31 of 50 transverse slabs of left patellae from a random series of 50 necropsy subjects aged 18 to 96 years, Evidence is presented that the splits are not due to autolysis nor to technical artefact, and it is here suggested that they result from shearing damage at the uncalcified-calcified cartilage interface during life.