We studied the reliability of extension-flexion radiography by analysing intra-observer and inter-observer variations in measurements of 30 patients with established L5-S1 spondylolisthesis. The highest intra-observer angular variations among three radiologists were found at the L5-S1 level (mean 1.6 degrees, S.D. 1.6 degrees, max. 9 degrees) and the highest sagittal translation also at the L5-S1 level (mean 0.6 mm, S.D. 0.8 mm, max. 4 mm). The highest angular inter-observer variation was found at the L5-S1 level (mean 2.6 degrees, S.D. 2.3 degrees, max. 11 degrees) while the highest variation in sagittal translatory movement was found at the L4-L5 level (mean 1.4 mm, S.D. 1.2 mm, max. 6 mm). The mean intra-observer variation for L5 olisthesis was 1.0 mm (S.D. 0.9 mm, max. 5 mm) and the corresponding inter-observer variation 1.3 mm (S.D. 1.1 mm, max. 6 mm). Thus, in general, the overall consistency and concordance were good. The variations in intra-observer and inter-observer readings were similar. Variability in readings did not depend on the magnitude or abnormality of the readings. On the other hand, variability may occur because of difficulty in defining the exact landmarks used for the measurements. We point out that in certain cases the diagnostic value of one single reading of an extension-flexion radiography examination may be questionable.