Test results from a size effect study on axially loaded reinforced concrete columns are presented. The tests specimens were geometrically similar pin-ended concrete columns of different sizes (in the ratio 1:2:4) giving slenderness ratios of 9.7, 18.0, and 34.7. The columns had square cross sections of sides 50, 100, and 200 mm, and varied in length from 0.14 m to 2.08 m. It was observed that for all slenderness ratios, the failure loads exhibited a size effect in which the nominal stress at maximum load (failure load divided by cross-sectional area) decreased as the size was increased. This contradicts the current design codes, which make no allowance for such size effect, and indicates that the failure is governed by fracture mechanics. The results are in good agreement with the size-effect law previously proposed by Bazant. The imperfections introduced by the loading arrangement were analyzed using the Southwell plot method. The test results showed that larger size columns exhibited larger imperfections.