Objective: To investigate the effect of initial insulin dosage on blood glucose (BG) dynamics, -cell protection, and oxidative stress in type 1 diabetes mellitus. Methods: Sixty newly diagnosed type 1 diabetes mellitus patients were randomly assigned to continuous subcutaneous insulin infusions of 0.60.2IU/kg/d (group 1), 1.00.2IU/kg/d (group 2), or 1.4 +/- 0.2IU/kg/d (group 3) for 3 wk. BG was monitored continuously for the first 10d and the last 2d of wk 2 and 3. A total of 24-hour urinary 8-iso-PGF2 alpha was assayed on days 8, 9, and 10. The occurrence and duration of the honeymoon period were recorded. Fasting C-peptide and glycosylated hemoglobin (HbA1c) were assayed after 1, 6, and 12 months of insulin treatment. Results: BG decreased to the target range by the end of wk 3 (group 1), wk 2 (group 2), or wk 1 (group 3). The actual insulin dosage over the 3wk, frequency of hypoglycemia on wk 1 and 2, and median BG at the end of wk 1 differed significantly, but not 8-iso-PGF2 alpha and the honeymoon period in the three groups. No severe hypoglycemia event was observed in any patient, but there was significant difference in the first occurrence of hypoglycemia. Conclusions: Differences in initial insulin dosage produced different BG dynamics in wk 1, equivalent BG dynamics on wk 2 and 3, but had no influence on short- and long-term BG control and honeymoon phase. The wide range of initial insulin dosage could be chosen if guided by BG monitoring.