The effects on postprandial glycaemic reactions of adding a glass of water to a meal were studied in 7 healthy male subjects and 20 Type 2 (non-insulin-dependent) diabetic patients for a period of up to 3 h. The subjects were served a meal of potatoes and meat, with or without 300 ml of water, in random order on two mornings after a 12-h fast. The diabetic patients were considered as well-controlled or not well-controlled according to HbA1c and blood glucose fasting values. Water addition increased the peak blood glucose (p less than 0.02) and serum insulin (p less than 0.02) levels in healthy subjects, and the blood glucose concentration in well-controlled diabetic patients (p less than 0.02). The addition of water also increased the overall blood glucose response, calculated as the positive incremental area, in healthy subjects by 68 +/- 25% (p less than 0.02) and in well-controlled diabetic patients by 40 +/- 14% (p less than 0.01). In poorly-controlled diabetic patients, however, the addition of water did not display significant effects, probably due to the varying fasting glycaemia in these patients. Thus, altering the physical property of a meal by dilution with water can affect the physiological responses; the results are considered to be relevant for the on-going discussion concerning the use of physiological responses to foods as a basis for diet instructions to diabetic patients.