Objective: To examine whether students use or avoid newly shaded areas created by shade sails installed at schools. Design: Cluster randomised controlled trial with secondary schools as the unit of randomisation. Setting: 51 secondary schools with limited available shade, in Australia, assessed over two spring and summer terms. Participants: Students outside at lunch times. Intervention: Purpose built shade sails were installed in winter 2005 at full sun study sites to increase available shade for students in the school grounds. Main outcome measure: Mean number of students using the primary study sites during weekly observations at lunch time. Results: Over the study period the mean change in students using the primary study site from pre-test to post-test was 2.63 (95% confidence interval 0.87 to 4.39) students in intervention schools and 0.03 (1.16 to 1.09) students in control schools. The difference in mean change between groups was 2.67 (0.65 to 4.68) students (P=0.011). Conclusions: Students used rather than avoided newly shaded areas provided by purpose built shade sails at secondary schools in this trial, suggesting a practical means of reducing adolescents' exposure to ultraviolet radiation. Trial registration: Exempt.