The results from experimental and epidemiologic studies suggest higher risks for certain gynaecological and gastrointestinal cancers in persons with diabetes. The strongest associations between diabetes and cancer risk have been observed for individuals with type 2 diabetes, while similar tendencies have been reported with respect to prediabetes, gestational diabetes and type 1 diabetes. In addition to other obesity-related mechanisms such as chronic inflammation, an upregulated insulin–IGF1 axis (IGF: insulin-like growth factor) and hyperglycaemia may underlie such associations. Data from cohort studies indicating lower cancer risks under metformin vs. insulin therapy have to be interpreted with caution, and randomized trials on potential anticancer effects of metformin among cancer patients with and without diabetes are ongoing. Generally, lifestyle interventions and successful drug therapy in early phases of diabetes development are considered as important means of cancer prevention.