  Babai and S\'os have asked whether there exists a constant c>0 such that
every finite group G has a product-free subset of size at least c|G|: that is,
a subset X that does not contain three elements x, y and z with xy=z. In this
paper we show that the answer is no. Moreover, we give a simple sufficient
condition for a group not to have any large product-free subset.
