Diet and exercise could be an essential part of the treatment of non-insulin dependent diabetes mellitus (NIDDM). The effects of a strict dietary restriction (800–1,200 Kcal/day) with exercise (2-hour walk per day) on glycemic control were evaluatedin NIDDM patients. A short-term study was performed on 147 cases In these cases, the duration of hospitalization was 7–14 days. We achieved glycemic control [fasting blood sugar (FBS) less than 140 mg/dl] in 112 cases (76%). Among them, 78 (53%) were controlled with diet and exercise only and in 34 (23%), control was induced with oral gliquidone treatment for about 5 days A long-term study was done on 76 cases who followed our program for from 1 to 12 months (mean : 3.8 months) after discharge. Glycemic control was maintained in 56 (74%) in spite of the insignificance of the amount of weight reduction. Glycemic control was significantly related to the duration of diabetes, diabetic complications and the peak C-peptide level, but was unrelated to the initial body weight, FBS levels and HbA1c levels. These data indicate that a program of diet and exercise must be an integral part in the treatment of NIDDM. This was true, especially, for patients who have had adequate insulin secretory capacity few diabetic complications, short duration of disease, and no previous history of oral hypoglycemic agents or insulin therapy.