The purposes of this study were to assess 1) the intertester reliability of goniometric measurements at the knee and 2) the validity of the clinical measurements by comparing them to measurements taken from roentgenograms. Thirty healthy subjects between the ages of 20 and 60 years were studied. The subjects were positioned on their right side on a roentgenographic table with their left lower extremity on a stabilizing board that was elevated 15 cm above the table's surface. For standardization of the position, an assistant placed the posterior aspect of the subject's left thigh in contact with two 15-cm pegs, which had been inserted perpendicularly into the stabilizing board. The assistant then moved the left leg to achieve an arbitrary angle of the knee joint and held the limb in that position. Two physical therapists then independently used a standard plastic goniometer to measure the knee joint angle in the sagittal plane using the greater trochanter, the lateral condyle of the femur, the head of the fibula, and the lateral malleolus as bony landmarks. A roentgenogram was taken of the extremity before the subject was moved. Pearson product-moment correlation coefficients (r's) and intraclass correlation coefficients (ICCs) were used to analyze the data. The data analysis revealed that the intertester reliability (r = .98; ICC = .99) and validity (r = .97-.98; ICC = .98-.99) were high. The results of this study indicate that goniometric measurements of the knee joint are both reliable and valid.