Type 2 diabetes is associated with raised risk of several cancers, but for type 1 diabetes risk data are fewer and inconsistent We assembled a cohort of 23 473 UK patients with insulin‐treated diabetes diagnosed at ages <30, almost all of whom will have had type 1 diabetes, and for comparison 5058 diagnosed at ages 30 to 49, of whom we estimate two‐thirds will have had type 2, and followed them for an average of 30 years for cancer incidence and mortality compared with general population rates. Here, INS is linked to diabetes mellitus.