Twenty-five children aged 2-14 years (mean age 8.39 +/- 0.78 years) were studied prospectively during the first year after the diagnosis of type 1 diabetes. Of their clinical and metabolic features at diagnosis, only age showed a significant independent relationship with endogenous C-peptide production during the first year. Age was correlated with higher values for basal and stimulated plasma C-peptide at 7-14 days after diagnosis, at 6 months and at 12 months. At diagnosis, age was also associated with a higher value for HbA1c and a lower prevalence of insulin antibodies. C-peptide production peaked at 3 months and thereafter declined. Mean HbA1c and insulin requirement were both minimal at 6 months. At diagnosis, there were significant inverse relationships between basal C-peptide production and both insulin dose and HbA1c and between stimulated C-peptide production and HbA1c. Basal and stimulated C-peptide production were inversely related to insulin dose at 6 and 12 months. Stimulated C-peptide was higher at 12 months in children retaining islet cell antibodies. These findings confirm the importance of age as a predictor of residual beta-cell function in type 1 diabetes and indicate that older children present clinically following a slower course of beta cell destruction.