Cancer in a mammal can be treated or prevented by administering to a mammal in need thereof a cancer inhibiting amount of metabolite(s) of .alpha.-linolenic acid, such as stearidonic acid (18:4 n-3), eicosatetraenoic acid (20:4 n-3), docosaptentaenic acid (22:5 n-3) and mixtures thereof, especially metabolites including stearidonic acid.