The opening contraction energy (Eop), i.e. the energy expended by the detrusor in the opening phase of micturition, was measured in 70 males (mean age 63 ± 9 years) with bladder outflow obstruction (compressive obstruction) due to benign prostatic hypertrophy. Significant correlations (p < 0.001) were found between Eop and the maximum external voiding power developed by the detrusor, Wext max (rs = ––0.57), between Wext max and the fraction of bladder volume voided, VV% (rs = 0.65), and between Eop and VV% (rs = ––0.77) – which suggests that increasing values of Eop affect the energy which the detrusor can supply for completion and termination of voiding. It then seems the parameter Eop would be able to assess not only the opening phase of micturition, but the emptying phase as well – the greater Eop (i.e. the greater the work needed to unfold the urethral lumen and start urine flow), the lesser the muscle mechanical capability of the urinary bladder to completely void. Thus, we eventually believe that calculating the parameter Eop may prove to be a complete, though indirect, method of quantifying the degree of prostatic (compressive) obstruction of the vesical outlet by a single num