Summary—Forty females with urinary stress incontinence (mean age 52 ± 10 years) and 20 age‐matched controls (mean age 49 ± 15 years) underwent complete urodynamic evaluation. In the former group, all patients complained of irritative bladder symptoms (frequency, nocturia, urgency) and 20 of them (Group B) showed detrusor instability (DI); in the other 20 (Group A) a final urodynamic diagnosis of genuine stress incontinence (GSI) was made. Instability was not apparent in Group B at standard filling cystometry, but had to be elicited through aggressively stressing the bladder by provocation tests.In all women the acceleration of flow rate (±, in ml/s2) was calculated from free flow uroflowmetry as the ratio of peak flow divided by the time to peak flow. This parameter did not differ significantly in GSI patients when compared with controls but proved higher in the DI group than in either the controls or the GSI patients.It was concluded that measuring the acceleration of flow rate is a simple and reliable test which improves the accuracy of diagnosis of detrusor instability in stress incontinent