The prevalence of bladder outlet obstruction in women is unknown and most probably has been underestimated. Moreover, there are no standard definitions for the diagnosis of bladder outlet obstruction in women. Our study was conducted to define as well as to examine the clinical and urodynamic characteristics of bladder outlet obstruction among women referred for evaluation of voiding symptoms. Bladder outlet obstruction was defined as a persistent, low, maximum "free" flow rate of <12 mL/s in repeated non-invasive uroflow studies, combined with high detrusor pressure at a maximum flow (p(det.Q)(max) >20 cm H(2)O) during detrusor pressure-uroflow studies. A urodynamic database of 587 consecutive women identified 38 (6.5%) women with bladder outlet obstruction. The mean age of the patients was 63.9 +/- 17.5 years. The mean maximum "free" flow, voided volume, and residual urinary volume were 9.4 +/-3.9 mL/s, 144. 9 +/- 72.7 mL, and 86.1 +/- 98.8 mL, respectively. The mean p(det. Q)(max) was 37.2 +/- 19.2 cm H(2)O. Previous anti-incontinence surgery and severe genital prolapse were the most common etiologies, accounting for half of the cases. Other, less common, etiologies included urethral stricture (13%), primary bladder neck obstruction (8%), learned voiding dysfunction (5%), and detrusor external sphincter dyssynergia (5%). Symptomatology was defined as mixed obstructive and irritative in 63% of the patients, isolated irritative in 29%, and isolated obstructive in other 8%. In conclusion, bladder outlet obstruction in women appears to be more common than was previously recognized, occurring in 6.5% of our patients. Micturition symptoms relevant to bladder outlet obstruction are non-specific, and a full urodynamic evaluation is essential in making the correct diagnosis and formulating a treatment plan.