PURPOSE: To investigate the mechanism and type of aspermic ejaculatory dysfunction resulting from high-energy transurethral microwave thermotherapy of benign prostatic hyperplasia. MATERIALS AND METHODS: Eleven patients who complained of dry ejaculation longer than 12 months after transurethral microwave thermotherapy were examined. Transrectal ultrasonography as used to assess seminal vesicle or ejaculatory duct dilatation, urethroscopy to identify any ejaculatory duct orifice obstruction, and urinalysis after ejaculation to detect sperm as an indicator of retrograde ejaculation. RESULTS: Four of eight patients with no sperm on urinalysis after ejaculation showed both seminal vesicle dilation and ejaculatory duct orifice obstruction with or without verumontanal scar formation. Another three patients showed either seminal vesicle dilation or ejaculatory duct orifice obstruction, and the final patient with no sperm on urinalysis showed neither seminal vesicle dilation nor ejaculatory duct orifice obstruction. Five patients with no sperm on urinalysis after ejaculation complained of some combination of perineal, scrotal, and urethral discomfort during or immediately after ejaculation. Three patients had sperm in their urine after ejaculation. CONCLUSIONS: In 7 patients (63.6%), dry ejaculation resulted from ejaculatory duct obstruction. Three patients (27.3%) had retrograde ejaculation, and one patient (0.09%) may have had dry ejaculation as a result of failure of spermatogenesis. The main mechanism of dry ejaculation after transurethral microwave thermotherapy seems to be ejaculatory duct obstruction, not failure of bladder neck closure.