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Objectives To assess prepubertal boys with posterior urethral valves (PUV) using an analysis of pressure-flow studies to evaluate the voiding phase and thus determine if myogenic failure (hypocontractility) arises before puberty and if it can be detected early. Patients and methods Eleven boys (8–13 years old) with PUV underwent urodynamics and the results were analysed using pressure-flow mathematical analysis (PFA) of the following variables of detrusor activity: contraction velocity (Vdet ), detrusor contractile power expressed as power factor (WF) and Schafer’s diagram, which differentiates a ‘strong’, ‘normal’ and ‘weak’ detrusor. Vdet and WF were compared with normal values previously determined in boys of similar age and considered ‘low’ if more than 2 SDs below the mean. The results of PFA were compared with standard pressure-flow studies and the three classical urodynamic patterns in boys with PUV, as determined by voiding symptoms. The subsequent PFA of seven of the 11 boys were also assessed as they had undergone previous urodynamics when <8 years old. Results As assessed by the three patterns of dysfunction, two boys had bladder instability, two had low compliance and three had hypocontractility, with four boys being normal. From the PFA, the Vdet and WF were lower than normal, respectively, in seven and nine of the 11 boys; Schafer’s nomogram showed a ‘weak’ detrusor in seven boys. The PFA suggested a pathology in four of five boys with symptoms and in three of six with no symptoms (two of the six showing a ‘low’ WF). Moreover, in older (11–13 years) boys, all five had a ‘weak’ detrusor, a ‘low’ WF and four a ‘low’ Vdet . Of the seven patients who underwent repeat PFA, three had a stable WF 3 years later, one (normal) worsened slightly and two were clearly worse, while one, who underwent late (at 3 years old) valve ablation, had an increased WF. Conclusions The PFA showed hypocontractility in two-thirds of prepubertal boys with PUV, including asymptomatic patients. These findings confirm the hypothesis that bladder dysfunction in boys with PUV eventually causes detrusor myogenic failure and finally a postpubertal overdistended bladder. [ABSTRACT FROM AUTHOR]