In six intact cadaver knees, we measured how the distance between six selected points in and around the femoral and tibial attachment area of the posterior cruciate ligament (PCL) changed with knee flexion. After complete removal of the PCL, 2-mm drill holes were made at the selected points. Each femoral point was measured against each tibial point using a heavy string that was passed through the drill holes. The distal end of the string was attached to a measuring unit. The changes in femorotibial distance were noted during flexion from 0 degrees to 110 degrees in 10 degree steps. The tibial drill hole locations had only a minor effect on the changes in femorotibial distance. The most isometric point was located in the centre of the posterior intercondylar area. The femoral locations of the drill holes were the primary determinant of whether the distance increased, decreased or remained nearly constant. According to our results, the most isometric femoral point is located at the posterosuperior margin of the anatomical PCL attachment. Using the tibial isometric point as a reference, the femoral points positioned anterior or posterior to the isometric point produced considerable changes in the femorotibial distance upon knee flexion. The anterior point led to an increase of about 7-8 mm at 110 degrees of flexion, the posterior point to a decrease of the same extent. Much smaller changes in femorotibial distance resulted from the points located superior or inferior to the femoral isometric point.(ABSTRACT TRUNCATED AT 250 WORDS)