Objectives: To determine the prevalence and severity of, and risk factors for, daytime urinary incontinence in children starting primary school. Design and setting: Population-based cross-sectional survey of new entrant primary school children in Sydney, Australia. Methods: A random cluster sample of 2020 primary school children was surveyed by using a daytime incontinence questionnaire with known substantial repeatability (mean kappa = 0.70). Results: The questionnaire was returned for 1419 (70%) children with a mean age of 5.9 years; 16.5% of children had experienced one or more episodes of wetting in the last 6 months (mild), 2.0% had wet twice or more per week (moderate), and 0.7% were wet every day (severe) (overall prevalence of 19.2%). On multivariate analysis, recent emotional stress (odds ratio 5.7), a history of daytime wetting along the paternal line (odds ratio 9.3), and a history of wetting among male siblings (odds ratio 5.3) were independent risk factors for moderate to severe daytime wetting. Expressed as population attributable risk, 59% and 28% of moderate-severe and mild daytime wetting, respectively, can be attributed to these 3 factors. Only 16% of families with affected children had sought medical help. Conclusions: Daytime urinary incontinence in the first year of primary school is more common than previously reported, and only a small proportion of affected children seek medical help. Emotional stress and family history are likely to be major causal factors.