Analysis of 185 consecutive patients with BPH revealed concomitant detrusor instability in 20% of the patients. Of all patients 30.9% were not obstructed, 51.8% were moderately obstructed and 17.3% were severely obstructed. Patients with detrusor instability during filling cystometry revealed no differences in average age, prostate volume or symptoms. Mean filling cystometry parameters revealed earlier sense of urge correlating with higher pressures at lower volumes in patients with detrusor instability. Pressure-flow analysis showed no differences between the patients with and without detrusor instability in bladder outflow obstruction parameters. Further analysis, however, revealed that the prevalence of patients with detrusor instability reaches a "steady state" at a moderate level of obstruction. On clinical epidemiological grounds, the conclusion is made that detrusor instability is developing in the early phase of obstruction. Probably detrusor instability and bladder outflow obstruction are concomitant, due to the aging process in many of these patients.