With the development of the C‐peptide assay more systematic studies of the endogenous insulin secretion have been possible in insulin‐dependent diabetes. Several reports now support the view that nearly 100% of patients have functioning beta‐cells at onset of disease. After start of insulin therapy most patients have a transient improvement in beta‐cell function, the remission period, with a maximum found between 2 and 5 months after diagnosis. Thereafter the beta‐cell function declines, but after 2 years duration of disease nearly 100% of the patients still will display residual beta‐cell function. The following years the prevalence of residual beta‐cell function will decrease to about 15% after 10 to 15 years duration, and thereafter no further decline in prevalence seems to occur. At present it is unclear why insulin‐dependent patients have different degree of beta‐cell function after years duration of disease. We have demonstrated that age at onset is of importance the first 10–15 years of disease, where the prevalence was highest in the elder patients. Thereafter the age at onset is without influence on prevalence of residual beta‐cell function. The degree of metabolic control is also of importance for beta‐cell function. Both at onset and in patients past their initial remission period has improvement in glucaemic control resulted in an improved beta‐cell function; but disappointingly this effect has only been transient. Whether poor metabolic control accelerates the destruction of the beta‐cells is not known. The importance of the many abnormal immunological parameters found in insulin‐dependent diabetics still has to be elucidated. Until now none of them have shown correlations to residual beta‐cell function. Therefore, at present it is impossible to predict at diagnose and during the first year of disease, which patients who will display residual beta‐cell function throughout life.