OBJECTIVES: Informed decisions about cancer screening require accurate knowledge regarding cancer risks and screening. This study investigates: (1) European women's knowledge of their risk of developing breast, ovarian, cervical or endometrial cancer, (2) their knowledge about mammography screening and (3) whether an evidence-based leaflet improves their knowledge. DESIGN: Cross-sectional online intervention survey. SETTING: National samples from five European countries (Czech Republic, Germany, UK, Italy and Sweden)-drawn from the Harris Interactive and the Toluna panel, respectively, in January 2017-were queried on their knowledge of age-specific risks of developing breast, cervical, ovarian or endometrial cancer within the next 10 years and of mammography screening before and after intervention. PARTICIPANTS: Of 3629 women (inclusion criteria: age 40-75 years) invited, 2092 responded and 1675 completed the survey (response rate: 61.4%). INTERVENTION: Evidence-based leaflet summarising information on age-adjusted female cancer risks, mammography and aspects of cancer prevention. PRIMARY OUTCOME MEASURES: Proportion of women (1) accurately estimating their risk of four female cancers, (2) holding correct assumptions of mammography screening and (3) changing their estimations and assumptions after exposure to leaflet. FINDINGS: Across countries, 59.2% (95% CI 56.8% to 61.6%) to 91.8% (95% CI 90.3% to 93.0%) overestimated their female cancer risks 7-33 fold (mediansacross tumours: 50.0 to 200.0). 26.5% (95% CI 24.4% to 28.7%) were aware that mammography screening has both benefits and harms. Women who accurately estimated their breast cancer risk were less likely to believe that mammography prevents cancer (p<0.001). After leaflet intervention, knowledge of cancer risks improved by 27.0 (95% CI 24.9 to 29.2) to 37.1 (95% CI 34.8 to 39.4) percentage points and of mammography by 23.0 (95% CI 21.0 to 25.1) percentage points. CONCLUSION: A considerable number of women in five European countries may not possess the prerequisites for an informed choice on cancer screening. Evidence-based information in patient leaflets can improve this situation.