PURPOSE: We investigated the clinical characteristics of IDDM patients, treated with NPH only, and evaluated current problems by measurement of serial blood glucose, insulin, C-peptide for 12 hours after administration of intermediate-acting insulin. METHODS: We studied 19 IDDM patients who had been diagnosed and followed up on a regular basis at Severance hospital. They were assigned into 2 groups, one(HbA1c high group) with HbA1c above 12%, the other(HbA1c low group) showing HbA1c below 12%. Their Heights, DM durations, HbA1c, basal C-peptides were primarily measured. Using continuous withdrawal pump, samples were taken every hour for 12 hours from 7:00 am. And serial blood glucose, insulin, C-peptide were assayed. RESULTS: 1) The mean HbA1c of the high group was 16.5+/-3.5% and that of the low group was 11.0+/-0.6%. There were no differences in clinical characteristics. 2) In HbA1c high group, fasting blood glucose, and mean blood glucose levels for 3hours were 156+/-85mg%, 284+/-125mg%(8,9,10am), 250+/-133mg% 11,12am,1pm), 252+/-122mg%(2,3,4pm), and 182+/-105 mg%(5,6,7pm), respectively. In low group, fasting blood glucose, and mean blood glucose levels for 3hours were 130+/-71mg%, 275+/-109 mg%(8,9,10am), 249+/-129mg%(11,12am,1pm), 231+/-81mg%(2,3,4pm), 158+/-62mg%(5,6,7pm), respectively. 3) Fasting blood insulin level was 51+/-47 U/l in high group, 62+/-62 U/l in low group. Thereafter low HbA1c group showed higher insulin levels than high HbA1c group. 4) Fasting blood C-peptide was 0.16+/-0.20 g/l in the high group, and 0.34+/-0.14 g/l in low group. Thereafter low group developed higher C-peptide responses than high group. The curve of C-peptide showed similar change of blood glucose, and maximal response followed 1-2 hours after maximal level of blood glucose. CONCLUSIONS: We concluded that short-acting insulin should be included for good control of blood glucose. Although fasting & dinner blood sugar seemed to be under fair control, intermediate-acting insulin used alone was not effective in preventing severe blood sugar elevation after morning meal.