Insulin dependent diabetes mellitus presenting in children under five years old exhibits several clinical and management features that differ from diabetes presenting in older children. In this review of the current population of the Oxford children's diabetes clinic, children with diabetes diagnosed aged 0- less than 5 years are compared with those diagnosed aged 5- less than 10 years to illustrate these differences. The mean annual age specific incidence of diabetes for children aged 0- less than 5 is 9.9/100 000 compared with 13.8/100 000 for the children diagnosed aged 5- less than 10. Although children with diabetes currently aged less than 5 comprise only 8% of the clinic population, such children ultimately make up 41% of the total number of children with diabetes aged under 15 attending the clinic. Diabetes diagnosed in children under the age of 5 seems to have increased in incidence over the past 10 years, exhibits a male preponderence (1.5:1), and shows an unusual seasonal variation in incidence with an autumn/early winter trough, late winter/early spring peak, and the absence of mid-summer trough seen in other age groups. First degree family history was positive in 16% of children diagnosed under the age of 5 compared with 10% of the group diagnosed aged 5- less than 10. In none of these children was the mother the affected relative.