A total of 68 men with bladder outflow obstruction and chronic retention (residual urine greater than 300 ml.) underwent investigation before and after prostatectomy with medium fill cystometry and natural fill long-term bladder pressure monitoring. Postoperatively, upper tract dilatation (present in 28 men preoperatively) resolved in all but 2 men and serum creatinine levels improved significantly. Irritative and obstructive symptom scores improved postoperatively (p less than 0.00006), although 17% of the men still had significant symptoms. Residual urine volumes decreased and flow rates improved (p less than 0.00006) but 32% of the men still had a residual urine of greater than 200 ml. Urodynamic parameters improved during medium fill cystometry and long-term monitoring. The main risk factors for upper tract dilatation are a pressure increase during bladder filling on conventional cystometry and the frequency of phasic detrusor activity during long-term monitoring, and they decreased postoperatively.