Background: Early insulin treatment is considered more beneficial than anti-diabetic medication with sulphonylureas, because the latter may exert negative effects on betacell function, while the former may help preserve it. In a previous study, we found that C-peptide response was increased in the insulin-treated group, whereas it was decreased in the glibenclamide group. However, it was not certain whether the advantage remained in the longer term. Aim: In this study, we tested whether early insulin treatment is more beneficial than glibenclamide against a 6-year follow-up perspective. Methods: We designed a randomized clinical trial in subjects with newly diagnosed type 2 diabetes. Glucagon stimulatory tests, measuring C-peptide and islet amyloid polypeptide (IAPP), were performed after 2, and 3, days of temporary insulin and glibenclamide withdrawal. Results: 18 subjects initially randomized to glibenclamide, and 16 randomized to two daily injections of insulin, participated in end-of-study investigations. C-peptide response to glucagon deteriorated (p < 0.01 vs. baseline) in initially glibenclamide-treated patients (n = 18), but not in insulin-treated patients (p < 0.05 for difference between groups, after 2 days of treatment withdrawal). The IAPP response to glucagon declined in the glibenclamide group (p < 0.001), but not in insulin-treated subjects (p = 0.05 for difference between groups). Conclusion: Early insulin treatment preserves beta-cell secretory function better than glibenclamide even in a 6-year perspective.