The existence of the plica synovialis infrapatellaris, which has long been known in anatomy, is explained from the point of view of evolution. The development of this formation in the early stages of human life is outlined too. The results of the analysis of 117 knee-joints lead to a classification respecting macroscopical points of view of the observed plicae synoviales infrapatellares. 47 plicae synoviales infrapatellares were examined by histological methods under the headings of material, blood vessels and nerves. Mechanical importance of the plica synovialis infrapatellaris must be denied in considering the results of X-ray examination. What is more, the plica synovialis infrapatellaris does not seem to have any importance for the nutrition of neighbouring areas, neither in pre- nor in postnatal life.