Lower urinary tract symptoms (LUTS) affect both men and women and can be caused by various conditions. Benign prostatic hyperplasia (BPH) is a frequent cause of LUTS in men and is a common histological finding — particularly in ageing men. Although ‘normal’ pros- tates in adult men are typically 15–30 ml in volume, and glands >30 ml are commonly deemed ‘enlarged’, no strict cutoff value has been defined and, for many physi- cians, enlarged prostate volume is a subjective finding on examination. Furthermore, the extent of prostatic enlargement varies considerably because the extent of hyperplasia is variable 1 . BPH is characterized by a proliferation of both stro- mal and epithelial cells of the prostate in the transitional zone surrounding the urethra 2 (FIG. 1). However, to date there is no evidence to suggest that men with LUTS due to BPH are at an increased risk of prostate cancer 3 . Furthermore, although common, BPH rarely causes death. Instead, the disease results in compression of the urethra, causing resistance to urine flow known as bladder outlet obstruction (BOO). This resistance can also result in obstruction-induced changes of bladder function, such as overactivity of the detrusor muscle or, conversely, reduced contractility of the detrusor muscle 2 . BOO can present as LUTS, infections or reten- tion, as well as other conditions. BPH and BOO impose considerable burden on the health of older men and on health care costs. As the world population ages, the incidence and prevalence of BPH and LUTS have rapidly increased 1 . Historical studies have examined non-modifiable risk factors — including age, genetics and geography — that have important roles in the aetiology of BPH and BOO 4 . However, several modifiable risk factors pres- ent new opportunities for treatment and prevention. In this Primer, we discuss the roles of these modifiable risk factors in BPH pathogenesis, including sex steroid hormones, the metabolic syndrome and cardiovascular disease, obesity, diabetes, diet, physical activity and inflammation. We also describe the current treat- ment paradigms and the future research trajectory of the condition. Epidemiology Approximately 50% of men >50 years of age will have pathological evidence of BPH, with this number increasing to >80% as men reach their eighth decade of life and older 5 . Furthermore, as men age, the likelihood of developing associated LUTS increases in a linear manner 6 (FIG. 2). However, many clinical epidemiological studies use different scales when measuring the severity of BPH depending on the defining terms, which can lead to differing data outcomes of the same disorder. The predictors of BPH progression and complications are outlined in BOX 1. For example, a community-based study from the Netherlands reported a 19% prevalence for BPH and LUTS in men 55–74 years of age when accounting only for a prostate volume of >30 ml and an International Prostate Symptom Score (IPSS) of >7 (REF. 7). The IPSS is a validated questionnaire to assess Correspondence to S.A.K. Department of Urology, Icahn School of Medicine at Mount Sinai, Mount Sinai Health System, 625 Madison Avenue, New York, New York 10022, USA. drprostate2@gmail.com Article number: 16031 doi:10.1038/nrdp.2016.31 Published online 5 May 2016 Benign prostatic hyperplasia Bilal Chughtai 1 , James C. Forde 1 , Dominique Dana Marie Thomas 1 , Leanna Laor 1 , Tania Hossack 3 , Henry H. Woo 3 , Alexis E. Te 1 and Steven A. Kaplan 2 Abstract | Benign prostatic hyperplasia (BPH), which causes lower urinary tract symptoms (LUTS), is a common diagnosis among the ageing male population with increasing prevalence. Many risks factors, both modifiable and non-modifiable, can increase the risk of development and progression of BPH and LUTS. The symptoms can be obstructive (resulting in urinary hesitancy, weak stream, straining or prolonged voiding) or irritative (resulting in increased urinary frequency and urgency, nocturia, urge incontinence and reduced voiding volumes), or can affect the patient after micturition (for example, postvoid dribble or incomplete emptying). BPH occurs when both stromal and epithelial cells of the prostate in the transitional zone proliferate by processes that are thought to be influenced by inflammation and sex hormones, causing prostate enlargement. Patients with LUTS undergo several key diagnostic investigations before being diagnosed with BPH. Treatment options for men with BPH start at watchful waiting and progress through medical to surgical interventions. For the majority of patients, the starting point on the treatment pathway will be dictated by their symptoms and degree of bother. NATURE REVIEWS | DISEASE PRIMERS VOLUME 2 | 2016 | 1 PRIMER ©2016MacmillanPublishersLimited.Allrightsreserved.