A few studies have suggested that the prevalence of diabetes is increased for patients with depression and for patients with bipolar disorder compared with the general population. However, no study has been published comparing the risk of getting a diagnosis of diabetes for patients with affective disorders with the risk for patients with other medical illnesses. It was the aim of the present study to investigate whether patients hospitalised for depressive or bipolar disorders are at increased risk of getting a diagnosis of diabetes at readmission compared to patients previously admitted for osteoarthritis. In a nationwide case register study, all patients who got a discharge diagnosis of depression, mania/bipolar disorder or osteoarthritis at first admission in a period from 1977 to 1997 were identified. The probability of getting readmitted and discharged with a diagnosis of diabetes was estimated with competing risks models in survival analysis. In total, 29,035 patients with a diagnosis of depression at first discharge, 6683 patients with mania/mixed episode and 108,525 patients with a diagnosis of osteoarthritis were identified. The risk of getting readmitted with diabetes was not increased for patients who had previously been admitted with depression or mania/bipolar disorder compared to patients with osteoarthritis. There was no difference in the risks of developing Type 1 and Type 2 diabetes.