We measured serum C-peptide, glucose, pH, islet antibodies and insulin antibody binding at diagnosis in 84 children with Type 1 (insulin-dependent) diabetes. In a subgroup of 33 children, residual insulin secretion (basal and peak C-peptide response to Sustacal), insulin antibody binding and HbA1c were measured at 10 days, 1, 3, 6 and 12 months. At presentation C-peptide correlated positively with age at onset and negatively with the blood glucose concentration. Median C-peptide concentration at diagnosis was low, rose significantly (p less than 0.05) at 10 days, reached a maximum at 1-3 months and declined gradually to 1 year. C-peptide concentration both at diagnosis and at 10 days correlated with that at 3 and 6 months. Of the factors investigated, only age (p less than 0.005) and sex (higher in females, p less than 0.01) were found to have a significant influence on basal/peak C-peptide levels throughout the first year. In particular there was no relationship between C-peptide, HbA1c and insulin dose during this period. A peak C-peptide response at 3-6 months greater than/less than 0.32 nmol/l was used to divide the group into two: 16 had a peak response less than 0.32 nmol/l (low secretors) while in 17, the peak C-peptide was greater than 0.32 nmol/l (high secretors). While the low secretors had significantly (p less than 0.05) lower C-peptide levels during the first year, there were no differences between low and high secretors in HbA1c or insulin dose.(ABSTRACT TRUNCATED AT 250 WORDS)