There is no consensus about a case definition of benign prostatic hyperplasia (BPH). In the present study, BPH prevalence rates were determined using various case definitions based on a combination of clinical parameters used to describe the properties of BPH: symptoms of prostatism, prostate volume increase, and bladder outflow obstruction. The aim of this study--in a community-based population of 502 men (55-74 years of age) without prostate cancer--was to determine the relative impact on prevalence rates of the inclusion of these different parameters (and of different cutoff values for these parameters) in a case definition of BPH. There is agreement that age is the dominant determinant of BPH. However, of 28 different case definitions that were formulated, only eight gave a statistically significant increase in the prevalence of BPH with age. The highest overall prevalence of 19% (95% confidence interval [CI], 15-23%) occurred using the definition that combines a prostate volume > 30 cm3 and an International Prostate Symptom Score (IPSS) > 7. The lowest prevalence rate of 4.3% (95% CI, 2-6%) occurred using the definition that combines a prostate volume > 30 cm3, an IPSS > 7, a maximum flow rate < 10 mL/s, and the presence of a postvoid residual urine volume > 50 mL. Thus, prevalence rates depend very much on the parameters used in a case definition. Follow-up will establish which men will eventually request a workup and treatment for BPH and will help determine the best clinical definition of BPH.