Objectives: To estimate the prevalence of overactive bladder (OAB) in Korean children, 5-13 years of age, and to assess the associated factors for OAB. Methods: A randomly selected cross-section study was conducted in 26 kindergartens and 27 elementary schools nationwide in Korea. A total of 19 240 children were included; a parent was asked to complete the questionnaires, which included items about OAB and voiding and defecating habits. OAB was defined as urgency with or without urge incontinence, usually with increased daytime frequency and nocturia (International Children's Continence Society, 2006). Its prevalence and associated factors were also investigated. Results: The response rate for the questionnaires was 85.84%. The overall prevalence of OAB was 16.59%. The prevalence of OAB decreased with age from 22.99% to 12.16% (P = .0001). The overall incidence of wet and dry OAB was 26.97% and 73.03%, respectively. Compared with normal children, those with OAB had a greater prevalence of nocturnal enuresis, constipation, fecal incontinence, urinary tract infection, delayed bladder control, and poor toilet facilities (P < .05). The incidence of increased daytime frequency and urge incontinence was 3.69% and 2.31% (P = .009) and 26.97% and 14.78% (P = .0001) in OAB and non-OAB children, respectively. The corresponding prevalence decreased with age from 5.04% to 3.06% and from 45.74% to 18.50% in OAB children (P = .0001). Conclusions: The overall prevalence of OAB in Korean children, 5-13 years of age, was 16.59% and decreased with age. Nocturnal enuresis, constipation, fecal incontinence, history of urinary tract infection, delayed bladder control, and poor toilet facilities might be factors associated with the development of OAB.