Abstract Background Increasing evidence suggests that diabetes mellitus (DM) is associated with increased cancer incidence and mortality. Several mechanisms involved in diabetes, such as promotion of cell proliferation and decreased apoptosis, may foster carcinogenesis. This study investigated the association between DM and cancer incidence and cancer-specific mortality in patients with breast and colorectal carcinoma. Methods A meta-analysis of controlled trials, prospective cohort studies and pooled cohort studies published after 2007 was conducted. Embase, PubMed and the Cochrane Library were searched. Summary hazard ratios (HRs) were calculated using a random-effects model. Sensitivity and subgroup analyses were performed to adjust for confounders, mode of DM assessment and follow-up time. Results Twenty studies were included to investigate the association between DM and breast and colorectal cancer incidence and cancer-specific mortality. The studies predominantly comprised patients with type II DM. The overall HR for breast cancer incidence was 1·23 (95 per cent confidence interval 1·12 to 1·34) and that for colorectal cancer was 1·26 (1·14 to 1·40) in patients with DM compared with those without diabetes. The overall HR was 1·38 (1·20 to 1·58) for breast cancer- and 1·30 (1·15 to 1·47) for colorectal cancer-specific mortality in patients with DM compared with those without diabetes. Conclusion This meta-analysis indicated that DM is a risk factor for breast and colorectal cancer, and for cancer-specific mortality.