Objective: The population of young people most vulnerable to low levels of physical activity (e.g. urban/minority/low socio-economic status/female/non-athletes) often has the least access to physical activity opportunities and resources. It has been suggested that a comprehensive, school-based approach, including prudent use of time before, during and after the school day may be an effective way to increase overall physical activity, but little is known about why urban students voluntarily participate in these extracurricular programmes. Using social cognitive theory as a framework, this study examines urban high-school students' rationale for attending after-school physical activity clubs designed to provide fun, safe, supportive places to do physical activities with friends. Design and setting: Qualitative study in an urban school district. Method: Over a 2-year period, adult leaders within 14 inner-city schools in a large urban district in the Midwest USA held 938 physical activity club sessions targeting traditionally inactive youth to promote non-competitive physical activity. Interviews with student participants (n = 278) and researcher field observations (n = 115) were used as primary data sources. Results: Three primary themes emerged to explain the voluntary participation of high-school students in after-school physical activity clubs: (a) social affiliation, especially group identification and establishing relationships with adult leaders and like-minded peers; (b) health and performance, including improving physical health and performance in other athletic endeavours; and (c) the "right" types of activities and focus, including those that were student-centred, non-competitive and culturally relevant. Conclusion: Creation and sustainability of successful inner-city physical activity clubs will largely hinge upon the capitalisation on an understanding of the motivations of an urban student population.