Forty patients with benign prostatic hyperplasia were evaluated with urodynamics and transrectal ultrasonography. Seventeen patients were found to have detrusor instability while the remaining 23 did not. No significant differences were noted during ultrasonography in the estimated prostatic volume, the presumed circle area ratio and the incidence of finding prostatic calcification between these two groups. However, the incidence of detecting intravesical protrusion of the prostate is significantly higher in patients with instability than in patients with stable bladder (53 vs. 13%, p < 0.01). It is therefore postulated that intravesical protrusion may increase afferent impulses from the prostate and alter the stability status of the urinary bladder.