Hjalgrim H, Frisch M, Ekbom A, Kyvik KO, Melbye M, Green A (Statens Serum Institut, Copenhagen: Odense University, Odense; Denmark: and University Hospital, Uppsala, Sweden). Cancer and diabetes - a follow-up study of two population-based cohorts of diabetic patients. J Intern Med 1997; 241: 471-5. Objectives. To study the occurrence of cancer amongst patients with diabetes mellitus (DM). Design. Population based cohort study. Setting. Denmark. Subjects. Two cohorts of patients with DM were identified. One cohort comprised 1659 conscripts diagnosed with type I DM before the age of 20 years. Another cohort comprised 1499 men and women with insulin treated DM identified by means of medical prescriptions on 1 July 1973. Both cohorts were followed until the end of 1992. Main outcome measures. The relative risk of cancer in the two cohorts was estimated as the ratio of observed to expected number of cancers in the cohort (SIR). Results. No unusual risk of cancer was observed amongst the conscripts (SIR 0.9, n = 13) or amongst patients with onset of DM before the age of 30 years in the prescription cohort (SIR 0.9, n = 32). Amongst those aged 30 years or more at DM onset in the prescription cohort, the overall risk of cancer did not depart from normal (SIR 1.0, n = 103), however, pancreatic cancer occurred in excess both immediately (< 1 year) (SIR 190, n = 1), and during 1-9 years after DM onset (SIR 9.0, n = 4). Similarly, the risk of non-Hodgkin's lymphoma was increased significantly (SIR 3.3, n = 6), all cases occurring more than 10 years after DM onset. Conclusions. Our data suggest that there is no unusual risk of cancer associated with type I DM. Type II DM may be the first symptom of pancreatic cancer and may be associated with an increased risk of non- Hodgkin's lymphoma.