We surveyed the clinical presentation, initial management and subsequent course of a prospectively registered, population-based cohort of 230 patients with Type 1 (insulin-dependent) diabetes mellitus diagnosed before age 21 years in the Oxford Regional Health Authority area in 1985 and 1986. Clinical details from the time of diagnosis were available on 219 patients. Thirty-four (16%) were in severe ketoacidosis with pH less than 7.10 or plasma bicarbonate less than 10 mmol/l, and 21 (10%) had mild to moderate ketoacidosis with pH 7.10-7.35 or plasma bicarbonate 10-21 mmol/l. One child died in ketoacidosis. Presentation in severe ketoacidosis was most common in children under age 5 years (p < 0.05), and ketoacidosis of any degree was less frequent in older children (0.05 < p < 0.01) and those with a parent or sibling with diabetes (p < 0.01). Within 4 years of diagnosis, 55 of 211 patients (26%) experienced severe hypoglycaemia, which in 31 (15%) led to one or more admissions. Readmission for unstable glycaemic control excluding acute hypoglycaemia occurred at least once within 1 year of diagnosis in 13% and within 4 years in 28%, and was more common in girls, in children aged less than 10 years at diagnosis, and those with a history of severe hypoglycaemia. A second cohort of 97 similar patients was recruited in 1990. The rates of admission at diagnosis (79%), severe ketoacidosis (13%) and mild to moderate ketoacidosis (13%) did not differ from the 1985/1986 cohort.(ABSTRACT TRUNCATED AT 250 WORDS)