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Test results obtained as part of a size-effect study on axially loaded plain concrete columns are presented. The test specimens were geometrically similar concrete columns (with dimensions in the ratio 1:2:4) having slenderness ratios of 9·7, 18·0 and 34·7. The columns were of square cross-section with sides of 50, 100 and 200 mm and varied in length from 145 mm to 2080 mm. Although such columns are at the bottom end of practical column sizes, the test results extend existing research information on size effects in columns. It was observed that for all slenderness ratios, the failure loads exhibited a size effect. The nominal stress at maximum load (maximum load divided by nominal cross-sectional area) decreased as the size of the columns increased. This behaviour is not taken into account in current design codes and needs further investigation to include practical column sizes. The results are not inconsistent with the 'size-effect law' proposed by Bazant.