The production of insulin autoantibodies (IAA) was studied after common viral infections in 12 children with type 1 diabetes mellitus and in their 18 healthy siblings. In addition, the production of IAA was measured after influenza vaccination with booster in 39 patients with type 1 diabetes mellitus and in 39 healthy controls. In 7 of the 12 diabetic children 13 viral infections were serologically confirmed. Among the siblings 14 periods of infection were noted in 9 individuals. A significant rise in IAA antibody titre was demonstrated in patients twice (IgG both times) and in siblings 11 times (IgM 5x, IgG 6x, difference significant P less than 0.05). In only three cases the rise in antibody titres occurred 6-12 wk after documented infection. There was a significant inverse correlation with age in both patients (r = 0.89, P less than 0.0001) and siblings (r = 0.67, P less than 0.001) for IgM IAA. After influenza vaccination a significant increase in IAA was noted twice: IgM IAA in a patient with diabetes and IgG IAA in a healthy volunteer. A four-fold decrease in IgG IAA was demonstrated in one diabetic patient. From these results it is concluded that IAA formation is not a direct sequela of viral infection or vaccination.