Background Elementary schools are one potential venue for sun protection interventions that reduce childhood sun exposure. Purpose To assess Year-2 results from a cluster randomized trial promoting hat use at schools. Design Block randomization was used to assign intervention/control status to participating schools. Data were collected from 2006 to 2008 and analyzed in 2007–2010. Setting/participants Of the 24 schools in the School District of Hillsborough County, Florida enrolled, 4th-graders were targeted in the first year and followed through their 5th-grade year. Intervention Classroom sessions were conducted to improve sun protection knowledge, foster more positive attitudes about hat use, and change the subjective norm of wearing hats when at school. Main outcome measures Year-2 outcomes assessed included hat use at school (measured by direct observation), hat use outside of school (measured by self-report) and skin pigmentation and nevi counts (measured for a subgroup of 439 students). Results The percentage of students observed wearing hats at control schools remained unchanged during the 2-year period (range 0%–2%) but increased significantly at intervention schools (2% at baseline, 41% at end of Year 1, 19% at end of Year 2; p <0.001 for intervention effect). Measures of skin pigmentation, nevi counts, and self-reported use of hats outside of school did not change during the study period. Conclusions This intervention increased use of hats at school through Year 2 but had no measurable effect on skin pigmentation or nevi. Whether school-based interventions can ultimately prevent skin cancer is uncertain. Trial registration This study is registered at ClinicalTrials.gov NCT 00346021.