The aim of this study was to clarify the effects of metabolic syndrome on benign prostatic hyperplasia (BPH) by use of transabdominal ultrasonography in healthy males, the subjects of this study were 333 Japanese men. All subjects were divided to two groups; 146 subjects in the smaller group (prostatic anteroposterior size<28.0 mm, mean age; 45.3yrs) and 187 subjects in the larger group (prostatic anteroposterior size ≥28.0 mm, mean age; 51.3yrs), respectively. The levels of age, systolic blood pressure (S-BP), diastolic blood pressure (D-BP), fasting insulin (FIRI), plasma triglyceride (TG), γ-glutamyltranspeptidase (γ-GPT), Fe, plasma prostate-special antigen (PSA), insulin resistance (HOMA-IR) and pancreas β cell function (HOMA-β) were significantly higher in the larger group than those in the smaller group, however plasma high density lipoprotein cholesterol (HDL-ch) level was significantly lower in the larger group than that in the smaller group. The prostatic anteroposterior size was significantly and positively related with age (r=0.31, p<0.0001), HOMA-IR (r=0.14, p=0.027), body mass index (r=0.10, p=0.049), S-BP (r=0.11, p=0.046) and D-BP (r=0.12, p=0.032) in all subjects. These data firstly indicated that metabolic syndrome was as a risk factor for developing BPH, secondly revealed that the transabdominal ultrasonography was able to use as the standard clinical tool for the useful assessment of prostatic size in daily medical health examinations.