One in 10 men in the developed world will present with prostate cancer (CaP), and in an ageing population developing strategies for its chemoprevention or treatment is of significance. For decades, androgen ablation has remained the frontline treatment for CaP that is no longer organ-confined and thus deemed surgically inoperable. Orchidectomy or drug-induced reduction of serum testosterone levels with the consequent removal of growth-promoting effects in the prostate is the driving rationale for this regimen. However, resistance often develops within a few months to years and androgen-insensitive tumours develop. In recent years, there has been an increasing focus on chemoprevention with agents such as finasteride being employed to reduce the risk of developing CaP. Significantly, such chemoprevention strategies are also based on 5α-reductase inhibition thus reducing intraprostatic dihydrotestosterone levels. Although there may be an overall reduction in CaP incidence in cohorts using such chemoprevention, in a subset of users who do develop this pathology there results a more aggressive, higher-grade disease. There have also been suggestions regarding the protective role of androgens against high-grade CaP. This leads to the intriguing notion that 17β-oestradiol (E2) may be an initiating driver of CaP; in fact, in old studies in which CaP was induced in rodents, E2 often accelerated the effect of the carcinogen. Might certain chemoprevention strategies or androgen ablation result in a systemic feedback loop in hormone synthesis or metabolism? If so, elevated serum E2 levels could result in its increased conversion to genotoxic catechol oestrogens in target tissues such as the prostate. Paradoxically, if E2 were to be an initiating factor in CaP, anti-oestrogens might be an overlooked treatment or chemoprevention strategy.