Insight into the natural history of beta cell function in IDDM patients obtained by C-peptide measurements is reviewed. It is argued that residual insulin secretion of metabolic importance is present in all IDDM patients during the initial course of the disease. After some months, beta cell function reaches its maximum; thereafter it declines at different rates dependent on the age at onset of diabetes and, possibly, on the presence of ICA and HLA-antigens. As many as 15% of IDDM patients retain life-long beta cell function that persists at approximately 10% of that observed in nondiabetic individuals. The residual endogenous insulin secretion is characterized by reduced capacity, as well as abnormal insulin secretory kinetics; these defects in residual insulin secretion can be modulated by changes in metabolic regulation as well as by immunosuppression during the initial course of the disease.