Introduction Socio-economic status (SES) is associated with both type 2 diabetes and cancer mortality and may confound their association.

Methods We performed a retrospective cohort study using a population-based national diabetes database for 35–84 year olds in Scotland for 2001–2007 and an area-based measure of SES. RRs for mortality from lung, colorectal, breast and prostate cancer among people with type 2 diabetes compared to the population without diabetes were estimated using Poisson regression.

Results Complete data were available for 210 994 eligible people (99.4%). There were 2081 deaths from lung cancer, 945 from colorectal cancer, 528 from breast cancer and 419 from prostate cancer. Age adjusted RR (95% CI) for lung, colorectal and prostate or breast cancer were 0.93 (0.82 to 1.04), 1.35 (1.23 to 1.48), 0.97 (0.89 to 1.07) for men and 1.03 (0.91 to 1.17), 1.34 (1.20 to 1.50), 1.49 (1.35 to 1.65) for women. Age and SES adjusted RR (95% CI) for lung, colorectal and prostate or breast cancer were 0.90 (0.84 to 0.96), 1.33 (1.22 to 1.47), 0.97 (0.88 to 1.07) for men and 0.98 (0.90 to 1.05), 1.34 (1.19 to 1.49), 1.49 (1.35 to 1.65) for women.

Conclusions Type 2 diabetes was associated with higher mortality from colorectal cancer and breast cancer. Adjusting for SES had a small effect on RR for lung cancer but almost no effect for other cancers suggesting that SES is not an important confounder of the association between type 2 diabetes and mortality from common cancers.