We report a pilot study to determine the preventive effect of small doses of insulin injected subcutaneously on slowly progressive β-cell damage in islet cell antibody (ICA)-positive patients with apparent NIDDM. Ten NIDDM patients who were ICA⁺ were divided into two groups of five. In the insulin group (age: 51 ± 8 years [mean ± SD], sex: 3 men and 2 women), intermediate-type insulin (3–16 U/day) was given once or twice daily as a subcutaneous injection. The sulfonylurea (SU) group (age: 48 ± 11 years, sex: 3 men and 2 women) was initially treated with a SU agent. Changes in β-cell function, as indicated by serum C-peptide responses and blood glucose values during a 100-g oral glucose tolerance test, as well as ICA and GAD antibody status, were evaluated for up to 30 months in both groups. ICA status became negative in four of five patients in the insulin group. ICA status did not become negative in any of the patients in the SU group (P = 0.047 vs. insulin group). ICA status was persistently positive in two patients whose β-cell function eventually progressed to an insulin-dependent state and fluctuated in the remaining three patients. In the insulin group, GAD antibody status became negative in one of four initially GAD antibody–positive NIDDM patients. In the SU group, GAD antibody status was persistently positive in three NIDDM patients (NS vs. insulin group). The serum C-peptide response improved significantly within 6 and 12 months in the insulin group, whereas it decreased progressively in the SU group. The changes in C-peptide response were significantly different between the two groups at 6, 12, 24, and 30 months. Two-hour blood glucose and HbA₁ values were unchanged in the insulin group, but they increased in the SU group. Subcutaneous small doses of insulin, resulting in a high rate of negative conversion of ICA and an improved serum C-peptide response, may be effective in treating ICA⁺ NIDDM patients who are at high risk for slowly progressive β-cell failure.