Background Low participation rates in randomised controlled trials involving children are almost a universal problem, leading to high cost and low statistical power. Trial, parent/family, child, and physician factors have been reported to influence parental willingness to consent for paediatric trials. Purpose To identify modifiable and unmodifiable factors associated with parental consent. Methods Demographic and clinical characteristics of children and their families and physician characteristics associated with parental consent were evaluated in a recent randomised placebo-controlled trial of prophylactic antibiotics to prevent recurrent urinary tract infection. Results Of 1109 eligible children identified (mean age, 2.0 years), 412 parents (37.2%) consented. On a multivariate analysis, the only modifiable factor associated with consent was request for consent by a member of the research study team rather than by a member of the clinical team (risk ratio (RR) = 1.9, 95% confidence interval (CI): 1.2-2.9). The unmodifiable factors significantly associated with consent were age of the child (≥4 years) (RR = 1.2, 95% CI: 1.1-1.4), presence of vesicoureteric reflux (RR = 1.5, 95% CI: 1.3-1.8), inpatient management of the index infection (RR = 0.8, 95% CI: 0.7-0.9), and multiple (-4) symptoms at presentation (RR = 1.3, 95% CI: 1.1-1.5). Limitations We have reported data from only one of the four participating centres in this trial. Data on non-consenters in other participating centres were not completely collected. Data on characteristics of the recruiting physician were limited. These findings are applicable for those considering a single randomised controlled trial. Conclusions Parent, child, and physician factors are associated with consent for trial participation, with most not being modifiable. Having a member of the research study team approach the parent for consent appears to be the only feasible strategy for increasing recruitment to randomised trials in this setting.