The quadriceps angle (Q-angle) is currently the only measurement of patellofemoral mechanics available in the clinical situation which does not necessitate the use of sophisticated radiographic equipment. It is generally assumed that for a given subject position and measurement procedure, the Q-angle is an absolute value rather than one which may vary with time. However, this assumption has never been tested. The purpose of this study was to determine whether the Q-angle varies with time. The Q-angle of 51 healthy subjects was measured continuously for one minute in the standing position using a video motion analysis system of proven accuracy and reliability. The Q-angle varied by an average of 3.12° (1.46°-6.97°) over the one minute tested, with a repeatability coefficient of 3.4°. The data indicate that when measured in the standing position the Q-angle is not an absolute measure, but one which varies with time. It is proposed that when the Q-angle is measured in the clinical situation with a goniometer, the value achieved is not a definitive measurement, but a snapshot in time of the individual's Q-angle within a given range.