OBJECTIVE To determine whether transvaginal ultrasound measurement of bladder wall thickness could replace ambulatory urodynamics when investigating women with lower urinary tract dysfunction not explained by conventional laboratory urodynamic studies. DESIGN A blinded prospective study. SETTING Tertiary referral unit in a London teaching hospital. POPULATION One hundred and twenty-eight women referred for ambulatory urodynamics with equivocal laboratory urodynamic findings or whose symptoms were not explained by the laboratory urodynamic findings. METHODS Transvaginal ultrasound assessment of bladder wall thickness was performed in three planes with an empty bladder prior to ambulatory urodynamics. Mean bladder wall thickness was calculated and the results analysed with respect to the ambulatory urodynamic diagnosis. MAIN OUTCOME METHODS Mean bladder wall thickness in women with a normal ambulatory study or a diagnosis of detrusor instability, genuine stress incontinence (GSI) or mixed incontinence. RESULTS Using a one way analysis of variance (ANOVA) bladder wall thickness was found to be significantly different in all diagnostic groups and this reached significance (P = 0.0001). There was no overlap in the 95% confidence intervals representing a diagnosis of detrusor instability or genuine stress incontinence. CONCLUSIONS Transvaginal ultrasound assessment of mean bladder wall thickness is a sensitive screening tool, which can detect detrusor instability in those women with equivocal laboratory urodynamics. In women who have no evidence of GSI on laboratory studies, a cutoff of 6.0mm is highly suggestive of detrusor instability. However, in those women with GSI then ambulatory studies probably remain the investigation of choice.