Abstract： Background: Work in voiding (WIV) of the bladder may be used to evaluate bladder status throughout urination rather than at a single time point.Few studies, however, have assessed WIV owing to the complexity of its calculations.We have developed a method of calculating work capacity of the bladder while voiding and analyzed the associations of bladder work parameters with bladder contractile function and bladder outlet obstruction (BOO).Methods: The study retrospectively evaluated 160 men and 23 women, aged ＞40 years and with a detrusor pressure at maximal flow rate (PdetQmax) of≥40 cmH2O in men, who underwent urodynamic testing.The bladder power integration method was used to calculate WIV;WIV per second (WIV/t) and WIV per liter of urine voided (WIV/v) were also calculated.In men, the relationships between these work capacity parameters and PdetQmax and Abrams-Griffiths (AG) number were determined using linear-by-linear association tests, and relationships between work capacity parameters and BOO grade were investigated using Spearman's association test.Results: The mean WIV was 1.15 ± 0.78 J and 1.30 ± 0.88 J, mean WIV/t was 22.95 ± 14.45 mW and 23.78 ± 17.02 mW, and mean WIV/v was 5.59 ± 2.32 J/L and 2.83 ± 1.87 J/L in men and women, respectively.In men, WIV/v showed significant positive associations with PdetQmax (r =0.845, P =0.000), AG number (r =0.814, P =0.000), and Schafer class (r =0.726, P =0.000).Conversely, WIV and WIV/t showed no associations with PdetQmax or AG number.In patients with BOO (Schafer class ＞ Ⅱ), WIV/v correlated positively with increasing BOO grade.Conclusions: WIV can be calculated trom simple urodynamic parameters using the bladder power integration method.WIV/v may be a marker of BOO grade, and the bladder contractile function can be evaluated by WIV and WIV/t.