The incidence of childhood diabetes was determined using the register of Leicestershire children with diabetes and from the population of Leicestershire. The register was commenced in the 1940s and has continued since then prospectively, being based on individual consultant records, specialist nurse records, and the Register of Births and Deaths. Accuracy was determined from hospital admission data and the U 100 insulin conversion register achieved in 1983. The incidence rates for all children of 0-14 years inclusive in the decade 1951-1960 was 3.84 patients per 100,000 per year. This increased to 5.34 during the period 1961-1970, and further increased in the years 1971-1980 to 10.6 patients per 100,000 per year. This was significant (p less than 0.05) using the 95% confidence interval of the odds ratio and very significant (p less than 0.001) by regression analysis (correlation coefficient r = 0.79). Statistically significant changes were found in each of the age groups 0-4, 5-9, and 10-14 years, and for girls and boys within those age groups (except boys aged 10-14 years).