Purpose For treating benign prostatic hyperplasia (BPH) 5 or 10 mg. terazosin hydrochloride daily has been routinely used in North America and Europe. We investigated the urodynamic effects of 2 mg. terazosin daily on Japanese patients with symptomatic BPH using pressure flow study. Materials and Methods A total of 20 Japanese patients 50 years old or older with symptomatic BPH underwent symptomatic and urodynamic evaluations, including pressure flow study, before and after terazosin treatment. Patients were given 1 mg. terazosin once daily for the first 7 days and they continued to receive 1 mg. terazosin twice daily for the following 3 weeks. Results At 4 weeks after terazosin treatment the International Prostate Symptom Score, quality of life index and maximum and average flow rates were significantly improved. Pressure flow study demonstrated decreased detrusor pressure at maximum flow, and minimum detrusor pressure during voiding and urethral resistance factor after terazosin treatment. Of the 20 patients 13 (65%) showed improvement in the linear passive urethral resistance relation. There was no significant difference in the maximum W. factor before and after terazosin treatment. Conclusions Terazosin treatment, even 2 mg. daily, urodynamically relieved bladder outlet obstruction in Japanese patients with symptomatic BPH without any changes in detrusor contractility.