BACKGROUND: Patients suffering from diabetes mellitus (DM) may experience an increased risk of cancer; however, it is not certain whether this effect is due to diabetes per se. OBJECTIVE: To examine the association between DM and cancers by a systematic review and meta-analysis according to the PRISMA guidelines. DATA SOURCES: The systematic literature search includes Medline at PubMed, Embase, Cinahl, Bibliotek.dk, Cochrane library, Web of Science and SveMed+ with the search terms: "Diabetes mellitus", "Neoplasms", and "Risk of cancer". STUDY ELIGIBILITY CRITERIA: The included studies compared the risk of cancer in diabetic patients versus non-diabetic patients. All types of observational study designs were included. RESULTS: Diabetes patients were at a substantially increased risk of liver (RR=2.1), and pancreas (RR=2.2) cancer. Modestly elevated significant risks were also found for ovary (RR=1.2), breast (RR=1.1), cervix (RR=1.3), endometrial (RR=1.4), several digestive tract (RR=1.1-1.5), kidney (RR=1.4), and bladder cancer (RR=1.1). The findings were similar for men and women, and unrelated to study design. Meta-regression analyses showed limited effect modification of body mass index, and possible effect modification of age, gender, with some influence of study characteristics (population source, cancer- and diabetes ascertainment). LIMITATIONS: Publication bias seemed to be present. Only published data were used in the analyses. CONCLUSIONS: The systematic review and meta-analysis confirm the previous results of increased cancer risk in diabetes and extend this to additional cancer sites. Physicians in contact with patients with diabetes should be aware that diabetes patients are at an increased risk of cancer.