The incidence of skin cancer has increased worldwide, with rates being especially high in Switzerland compared with other European countries. Extensive sun exposure during childhood is considered a key factor for skin carcinogenesis. The aim of the study was to evaluate the impact of a school-based sun safety education programme developed by the Swiss Cancer Leagues on primary school students’ sun-related knowledge, protective behaviours and sunburn rates. In summer 2011, 1-h sun safety education sessions were held at 33 primary schools throughout the Canton of Zurich (North-Eastern Switzerland). Children in the participating school classes (first, second and third graders) answered a questionnaire on their sun-related knowledge, behaviours and sunburn experience shortly before and 1 year after the intervention. Overall, 3110 completed pretest and 1738 post-test questionnaires were eligible for analysis. The evaluation of pretest data showed considerable room for improvement in terms of sun-related knowledge, considering that merely a good half of the children were conscious that the sun may present a hazard to health. Overall, more than 95% of students benefited from the protection of sunscreen (application by parents: 73%; application by child: 66%), but only 36% stated that they generally sought shade on sunny days. After the intervention, knowledge increased considerably and significantly (P<0.0001), but there was no change in sun-protective behaviours (use of sunscreen, seeking shade). However, we observed a nonsignificant trend towards decreased sunburn rates. The brief one-time sun safety education sessions were effective in sustainably improving children’s sun-related knowledge and possibly to some extent in decreasing their sunburn rates.