Childhood diabetes diagnosed before the age of 24 months presents specific management problems. We report here on the establishment (using the British Diabetic Association [BDA] Children's Register) and mortality of a cohort of children with diabetes diagnosed before age 24 months. Children registered during the period 1972-1981 were traced by contacting consultants or by using the National Health Service Central Registers (NHSCR) of the Office of Population Censuses and Surveys (OPCS). Standardized mortality ratios (SMR) were estimated using person-years of follow-up for each child and age-specific death rates for the England and Wales population for the years 1972-1989. Of 339 children notified during 1972-1981, 231 were traced through consultants and 99 of the remaining 108 through the NHSCR. Twenty were found to be ineligible. The cohort available for mortality analysis comprised 310 (97%) of 319 eligible children. Their age at the time the cohort was established was 8-18 years, and their duration of diabetes 8-17 years, The male:female ratio is 1.4:1. Of 310 children studied, seven have already died: SMR 5.4 (95% Cl : 2.5-11.5). We have established a large, unique cohort of children with diabetes diagnosed before age 24 months and still living in the UK and Ireland. The natural history including mortality and occurrence of complications will be analysed prospectively in this cohort and compared to other cohorts of similar disease duration, but later age at onset.