Oxidative stress is one of the new and most intriguing pathogenetic hypotheses of heart failure; it involves various mechanisms such as endothelial dysfunction, mechano-energetic uncoupling and apoptosis. Xanthine oxidase, a key enzyme in purine catabolism, is overexpressed in patients with heart failure, and it is also an important source of oxidizing activity molecules (free radicals, superoxide anion, oxygen peroxide, etc...). Allopurinol competitively inhibits the action of xanthine oxidase and effectively counters oxidative stress. It could thus prove useful in the treatment of heart failure: in fact it is the only drug that has been proven able to lower O2 consumption of dysfunctioning myocardium. The Authors briefly review the xanthine oxido-reductase enzyme system and in particular analyse the latest evidence reported in the literature on allopurinol in the treatment of heart failure.