We investigated the reproducibility of the various radiological methods of assessment of hip dysplasia by making 474 assessments of hips and quantifying the inter-observer and intra-observer variation. There was a wide range of variability between the readings made by different observers and by one observer on two occasions. A measurement of acetabular index has to be given a range of +/- 6 degrees in order to be 95% confident of including the true measurement. We found the most helpful measurements to be the acetabular index, up to the age of eight years; the centre-edge angle, over the age of five years; and Smith's c/b ratio and neck-shaft angle. We feel, however, that the change in value over a series of radiographs in the same child is much more valuable. Single readings of all the radiological measurements investigated in this study were unreliable.