Repeated ingestion of insulin has been suggested as an immune tolerization therapy to prevent immune-mediated (type 1) diabetes. We performed a placebo-controlled, two-dose, oral insulin tolerance trial in newly diagnosed (< 2 years) diabetic patients who had required insulin replacement for less than 4 weeks and were found to have cytoplasmic islet cell autoantibodies (ICAs). No oral hypoglycemic agents were permitted during the trial. Endogenous insulin reserves were estimated at six-month intervals by plasma C-peptide responses to a mixed meal. Positive ICAs were found in 262 (31%) of the 846 patients screened. Of the 197 who agreed to participate, 187 could be followed for 6 to 36 months. Endogenous insulin retention was dependent upon initial stimulated C-peptide response, age at diabetes onset, and numbers of specific islet cell autoantibodies found. Oral insulin improved plasma C-peptide responses in patients diagnosed at ages greater than 20 years, best seen at the low (1 mg/day) over the high (10 mg/day) insulin dose (P = .003 and P = .01, respectively). In patients diagnosed before age 20 years, the 1 mg dose was ineffective, whereas the 10 mg dose actually accelerated C-peptide loss (P = .003). There were no adverse effects. If confirmed, these findings suggest that diabetic patients over age 20 years with ICA evidence of late-onset immune-mediated diabetes should be considered for oral insulin at 1 mg/day to better retain endogenous insulin secretion.