To evaluate the effects of switching from NPH insulin(twice daily)to insulin glargine on blood glucose control(HbA1C),insulin dosage and frequency of hypoglycemia in Japanese patients with type 1 diabetes,we monitored these parameters in 37 type 1 diabetics for 12 months after switching.The patients were assigned to 3 groups according to HbA1C at switching :good(<7.0%),fair(7.0-7.9%)and poor(≥8.0%).Though there was no improvement in HbA1C in the good and fair groups over the 12 months,there was a significant decrease in HbA1C in the poor group(10.2±1.2%→9.2±1.2%,p<0.01).Over this period,total dosage and basal dosage significantly decreased as compared to the time of switching.In the good and poor control groups,there was a significant decrease in the frequency of hypoglycemia.In conclusion,insuling largine improves glycemic control in patients with type 1 diabetes mellitus,especially in patients with poor blood glucose control.