A sample of twenty-eight markets investigated by the UK Monopolies and Mergers Commission is taken and market share changes are analysed for subsequent erosion of dominance. The evidence points to a modest erosion for leading firms but only rarely to a loss of leadership. Although there is no significant difference between the rates of decline for leading firms in the MMC sample and a control group, the achievement of some erosion of dominance in markets where entry barriers were reduced may represent some limited success for competition policy.