Where are the welfare losses of imperfect competition large?

The welfare losses of imperfect competition have usually been analysed using detailed information about sales, costs and profits. We present a procedure for finding the potential welfare losses which needs only demand elasticity and an indicator of concentration. The models are based on Coutnot-Nash oligopoly, collusion and Stackelberg leadership, including cases where a price taking fringe exists. When using Finnish four digit industries as an example, it turns out that the potential deadweight losses may be a matter of concern.