The blood components of diabetic patients who visited S- hospital in seoul from January 1982 to June 1983 were compared with the reference levels. Hemoglobin and hematocrit levels of diabetic patients were significantly lower than the reference. The diabetic patients showed 2-3 times higher levels of fasting blood glucose and postprandial -2 hours blood sugar. Levels of blood urine nitrogen and creatine were also significantly higher in the diabetes mellitus and the levels of potassium sodium and chlorine showed no differences although these were decreased gradually in older patients. Lower serum calcium levels were seen in the diabetic patients and this change was more significant at the ages higher than 40. The effect of an active vitamin D on serum-Ca level in diabetic patients was studied in comparison to that of non- diabetic persons. The serum calcium levels were slightly increased in control and insulin- dependent diabetic patients after a week- intake of 1,000mg calcium a day, while the intake of 1,25-dihydroxycholecalciferol ( 1,000 IU per day ) did not increase the serum calcium levels of these groups. Insulin - independent diabetic patients showed the rather lower serum calcium levels after a week- intake of 1,000mg calcium per day. However, the levels were increased after 2-weeks intake of the calcium and a week-in-take of the active vitamin D(1,000 IU/day ). This effect of vitamin D was seen in the groups with lower intake of calcium(500mg/day ) but not in the groups with 1,000mg calcium intake a day.