The radiological assessment of knee deformity in the frontal plane is often fallacious because of the difficulty in determining the correct position of rotation of the lower limb. Deformity is under-estimated when the direction of the X-ray is not perpendicular to the frontal plane. To determine the frontal plane with accuracy the authors first looked for a true lateral view which is obtained when the posterior margins of the femoral condyles are superimposed. When the position of the patient has been determined by demonstrating the true lateral position, a second radiograph is taken using a large film in a plane at right angles to the preceding film. In this way a true frontal plane is obtained which requires no reference to the position of the foot or the patella which tends to be variable. An association flexion deformity does not invalidate the result. When this technique is used it can be shown that conventional methods tend to under-estimate the degree of knee deformity.