One of the causes of anterior knee pain may be symptomatic mediopatellar plica. The pain is usually attributed to the mechanical damage caused by the plica inside the joint, or, more precisely, to the synovitis induced as a result. Recent observations seem to suggest, however, that the pain is, in addition, likely to be engendered by an increased number of nerve-elements present in the substance of the plica. The present study used up-to-date histochemical methods to reveal in the symptomatic mediopatellar plicae nerve-elements that may be made responsible for the pain. Semi-quantitative methods were used to establish the number of nerve-elements in the tissue samples obtained from 21 symptomatic mediopatellar plicae (Group 'A'), exposing them to view with the aid of synaptophysin and neurofilament serum, coupled with routine light microscope as well as polarising microscope examinations following H&E and van Gieson staining. Tissue samples taken from the asymptomatic mediopatellar plica of 11 patients served as control (Group 'B'). A significantly larger number of nerve-elements were found in the substance of the plicae of Group 'A' 6.9 (S.D.+/-2.9) than in Group 'B' 3 (S.D.+/-1.2). Within Group 'A', more nerve-elements were revealed in trauma-related case histories than in those with no recollections of trauma (an average of 9.6 vs. 5.2, respectively). Similarly, the reduction of pain achieved by surgery was greater in the trauma-related group than in the non-traumatic one (3.0 vs. 1.8, respectively). Relying on our observations, we claim that the painfulness of the knee joint plicae is in all probability also attributable to the fact that their tissue substance contains an increased number of nerve-elements. A major trauma in a particular case history contributes, in all likelihood, to an increase in the number of nerve-elements, but further examinations are required to clarify the pathomechanism involved.