Blood glucose and serum insulin values were determined after oral glucose tolerance tests in 528 persons with suspected pathological or disturbed glucose tolerance. Results were evaluated depending on sex, age, overweight, and familial diabetes. There were no differences of serum insulin secretion and behaviour of blood sugar between males and females. With increasing age there was a marked decrease of insulin secretion. In contrast there was increased hyperinsulinaemia with increasing overweight. This was particularly evident in persons with comparably reduced glucose tolerance but with different body weight. "Classical" changes of insulin secretion, as described for prediabetes and manifest diabetes in contrast to persons with normal metabolism, were only seen in selected groups of probands with normal weight. Serum insulin shows greater variability than blood glucose. Estimations of insulin are not necessary when assessing disturbances of glucose tolerance.