Background Detrusor overactivity is one known cause of lower urinary tract symptoms and has been linked to bladder storage symptoms (urgency, frequency, or urge incontinence). Objective To determine clinical and urodynamic parameters associated with detrusor overactivity in patients with suspected benign prostatic hyperplasia. Design, Setting, and Participants During 1993–2003, urodynamic investigations were performed in patients aged 40 yr or older and with lower urinary tract symptoms, benign prostatic enlargement, and/or suspicion of bladder outlet obstruction (maximum flow rate <15 ml/s or postvoid residual urine > 50 ml). Measurements Detrusor overactivity was defined according to the new International Continence Society classification (2002) as involuntary detrusor contractions during cystometry, which may be spontaneous or provoked, regardless of amplitude. The Schäfer algorithm was used to determine bladder outlet obstruction. Results In total, 1418 men were investigated (median age: 63 yr) of whom 864 men (60.9%) had detrusor overactivity. In univariate analysis, men with detrusor overactivity were significantly older, more obstructed, had larger prostates, higher irritative International Prostate Symptoms Score subscores, a lower voiding volume at free uroflowmetry, and a lower bladder capacity at cystometry. The prevalence of detrusor overactivity rose continuously with increasing bladder outlet obstruction grade. Multivariate analysis showed that only age and bladder outlet obstruction grade were independently associated with detrusor overactivity. After age adjustment, the odds ratios of detrusor overactivity compared to Schäfer class 0 were 1.2 for class I, 1.4 for class II, 1.9 for class III, 2.5 for class IV, 3.4 for class V, and 4.7 for class VI. Conclusions In patients with clinical benign prostatic hyperplasia, detrusor overactivity is independently associated with age and bladder outlet obstruction. The probability of detrusor overactivity rises with increasing age and bladder outlet obstruction grade.