Environmental factors very early in life may be important for later development of insulin dependent diabetes. Because several of these factors, such as infections, vary with season, we predicted a difference in birth pattern compared with the general population among children who develop diabetes. In a population based study we analysed all 1248 children from seven paediatric departments in the south east part of Sweden to evaluate whether there is such a relation. There was a significant difference in birth pattern in patients with diabetes compared with the general population. Children who developed diabetes at the age of 10-15 years accounted for most of this difference. Boys had a more pronounced difference in birth pattern than girls. Children diagnosed with diabetes during years of high incidence, as well as children with an infection before diagnosis of diabetes, showed a significantly different birth pattern compared with the background population. These results indicate that there is a difference in birth pattern in children who develop diabetes compared with the background population. This supports the theory that environmental factors early in life play a role in the development of diabetes many years later.