Objective To assess whether the signs associated with specific signs (only vesico-ureteric reflux or vaginal voiding) in 25 (49%) of patients, whereas 26 (51%) detrusor instability (DI), as assessed by videourodynamic studies, can be evaluated by conventional had some signs suspicious of DI. Suspicious signs were urethral ballooning (in 11), bladder trabeculation or voiding cysto-urethrography (VCUG). Patients and methods Fifty-nine children who under-a constricting ring (eight), a 'spinning-top' urethra (three), urethral notching (two) and Mercier's bar went cystometry and VCUG were reviewed and divided into two groups; group 1 comprised 51 neurologically (one). In group 2, five patients had no abnormal findings on VCUG and three had suspicious signs. The normal children who had DI (47 girls and four boys, mean age 7.9 years, range 4-14), with no malfor-positive predictive value of VCUG was high (0.89) but the diagnostic yield of suspicious signs was low, mations or previous surgery. Most had mixed symptoms, including urinary tract infections (44) and because the sensitivity (0.5), specificity (0.62) and overall accuracy (0.52) were low. nocturnal enuresis with daytime symptoms (20); group 2 (control) comprised eight children (seven girls, Conclusion Radiological signs suspicious of DI cannot be regarded as such in conventional VCUG, as mean age 9.7 years, range 6-15) with a stable bladder and the same clinical presentation. The interval although they were detected in half the patients with DI, they also occurred in three of eight children with between VCUG and cystometry ranged from 1 day to 5 months (mean 47 days) and was similar in both a stable bladder. Keywords Bladder instability, children, voiding cysto-groups. Signs considered suspicious of DI were sought in the findings of VCUG.urethrography Results In group 1, VCUG was normal or showed no