Bladder outlet obstruction alters detrusor contractility, reducing the bladder's ability to respond to large filling with a risk of urinary retention. The objective was to assess the effect of bladder filling volume on detrusor contractility in men with bladder outlet obstruction. A prospective multicenter study in two pelviperineology departments. Male patients eligible for urodynamics (IPSS score > 7) were included from January to July 2022. In case of absence of bladder outlet obstruction on pressure-flow studies, they were secondarily excluded. The primary endpoint was the maximum isometric detrusor pressure during a stop-test, corresponding to detrusor contractility, measured at 3 filling volumes (50%, 75%, and 100% of cystometric capacity). Fifty-two patients performed urodynamics, of whom 12 were excluded because of lack of obstruction or inability to perform the stop-test. Detrusor contractility was significantly higher for a 75% bladder filling than 50% and for a 75% filling than 100%, with a mean difference of 19.5; confidence interval (CI) 95% [14.3; 24.8] and 12.2; CI 95% [6.9; 17.5] cmH2 O respectively (p < 0,01). In case of bladder outlet obstruction in men, detrusor contractility depends on bladder filling volume, with reduced contractility when the bladder was underfilled or overfilled. This phenomenon could help to explain the mechanisms of urinary retention in men with bladder outlet obstruction.