After an early report that patients treated with angiotensin-converting enzyme (ACE) inhibitors had a lower than expected incidence of cancers, there was a large number of publications investigating the possible pathophysiological mechanism mediating this effect, as well as population studies comparing the incidence of cancers in patients treated with agents inhibiting the renin-angiotensin system with their incidence in the general population. Several mechanisms are proposed to explain a potential anti-tumour activity of such agents in vitro in experimental animal models. However, the population studies are mostly inconclusive, although they do suggest a possible interaction between ACE genotypes and susceptibility to altered behaviour of certain tumours.