The International Continence Society Benign Prostatic Hyperplasia Study (ICS-BPH) was devised in response to the perceived underutilization of urodynamics in the diagnosis of benign prostatic obstruction (BPO), together with the apparent reliance on symptoms alone when selecting patients for invasive therapies, despite evidence that lower urinary tract symptoms (LUTS) had poor diagnostic specificity for BPO. The ICS-BPH Study is an international multicenter study employing a wide-ranging patient-completed questionnaire, with questions on symptoms, bothersomeness, quality of life, and sexual function. In addition, each patient underwent multiple urine flow studies and standardized urodynamic investigations (phase I). Phase II was an observational assessment of outcome following a range of treatments: conservative, medical, and surgical. Phase III included data from randomized controlled trials involving a wide range of therapies. One thousand, two hundred and seventy-one patients (mean age, 66 years) were included in phase I from 12 countries: 18% of patients were unobstructed, 22% mildly obstructed, and 60% obstructed on pressure flow studies; 83% of patients performed at least two flow studies. The ICS-BPH Study will allow definitive statements to be made concerning the diagnostic potential of a wide range of LUTS, and will define the place of urine flow studies, describe patients who will need pressure flow studies prior to invasive treatment, and identify those likely to experience an adverse outcome. Although an aim of the study is to produce a diagnostic symptom score, there is considerable doubt as to whether this can be achieved where aging and a variety of lower urinary tract dysfunctions often coexist.