It is not known whether early immunosuppressive treatment can preserve long-term endogenous insulin secretion in subjects with insulin-dependent diabetes mellitus. In the present study, clinical remissions during the first year and C-peptide production for 3 years were followed after 43 subjects with newly diagnosed insulin-dependent diabetes mellitus were randomly assigned to a cyclosporine A treatment group for 4 months or to a control group. Of the six cyclosporine A-treated subjects who had remissions, five were 19 years of age or younger, compared with two of the four in the control group. C-peptide production was present in 98% of all subjects after 4 months, in 88% after 1 year, and in 43% after 3 years. There were no significant differences in numbers of subjects with C-peptide production or in mean hemoglobin A1 levels, between cyclosporine A-treated and control subjects after 3 years. Cyclosporine A treatment of subjects with newly diagnosed insulin-dependent diabetes mellitus for a period of 4 months does not have the ability to preserve residual beta-cell function.