Objective: To estimate the number of cancers to be diagnosed in 2025 that could be prevented solely due to changes in diet and physical activity Design and setting: We used an Australian population-based cancer database to estimate the total number of cancers to be diagnosed in 2025, by applying published ageand sex-specific population projections to current cancer incidence rates, and multiplying the projected numbers of cancers by estimates of population-attributable fractions. Main outcome measures: Projected number of preventable cancers that would be diagnosed in 2025. Results: Our projections suggest that there will be about 170 000 Australians diagnosed with cancer in 2025. This represents an increase of about 60% on the 2007 incidence. Almost 43 000 of these cancers (low estimate, 42 295; middle, 42657; high, 43 990) could be prevented through improvements to diet and physical activity levels, including through their impact on obesity. It is likely that this is an underestimate of the true figure. The most preventable cancer types in 2025 were estimated to be bowel cancer and female breast cancer (10 049 and 7273 preventable cases, respectively). Conclusions: About 25% of cancers, or about 43 000 cancers in 2025, can potentially be prevented through improvements in diet and physical activity. It is imperative that governments, clinicians and researchers act now if we are to reduce the significant future human and financial burden of cancer.