To define the origin of urinary incontinence after radical prostatectomy urodynamic studies in 24 incontinent patients were compared to those of 13 continent patients. A statistically significant difference between incontinent and continent patients was found for the mean functional profile length (2.1 versus 3.6 cm., respectively, p less than 0.001), maximal urethral closure pressure (39 versus 74 cm. water, respectively, p less than 0.001) and maximal urethral closure pressure during voluntary contraction of the external sphincter (107 versus 172 cm. water, respectively, p less than 0.002). The differences among maximal detrusor pressure, initial bladder volume at which a detrusor contraction occurs, maximal cystometric capacity and residual urine were not statistically significant between the 2 groups. Urethral instability was present in 1 of the 24 incontinent patients (4.2%) and in none of the 13 continent patients, while detrusor instability was present in 6 (25%) and 3 (23.1%), respectively. Urethral and detrusor instability correlated poorly with incontinence (correlation coefficients 0.123 and 0.021, respectively). The appearance of the bladder outlet on voiding cystourethrography was correlated with urodynamic parameters and the presence or absence of continence. Tubularization above the level of the external sphincter was present in continent but absent in incontinent patients. Continence after radical prostatectomy is dependent upon sphincteric efficiency, which may be influenced by the anatomical configuration of the reconstructed bladder outlet and the integrity of the distal urethral sphincteric mechanism.