The accuracy of knee goniometry was examined by comparing goniometric measurements with radiographic bone angle measurements of six positions of the knee, namely, 0 degree, 15 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees. Within the first 15 degrees of knee flexion joint excursion measured by goniometry differed significantly from bone angle measurement (p less than 0.01). With 30 degrees of knee flexion or more, no significant difference was found between goniometric measurements and bone angle measurements of knee motion. These findings suggest that within the first 15 degrees of knee flexion, goniometric measurement of joint excursion may be remarkably wrong. On account of this, it was suggested that when range of motion is limited at the knee joint, caution should be exercised in determining movement gains or losses within the initial 15 degrees of knee flexion.