Despite being such a common condition, morphologically prevalent in 88% of autopsies of old (>80 years) men, little epidemiologic research has been undertaken on benign prostatic hyperplasia (BPH). The prevalence of BPH, and probably the incidence, increases with age although the belief that BPH is a direct consequence of aging per se still awaits proof. Many observers have concluded that the age association reflects age-related hormonal changes, although this also requires proof. BPH is a very common condition in aging men: 3 in every 10 may ultimately require surgery for this condition if current estimates of prevalence are correct. Despite its common occurrence, little is known with any degree of certainty about risk factors for BPH apart from being male, being old, and having had a pair of functioning testicles since puberty. There are good reasons why the epidemiology of BPH has remained poorly understood although the application of more epidemiologic thought could pay great dividends, particularly if prostate screening programs could be exploited maximally.