HbA_1c was measured in 3240 healthy non-diabetic adult individuals in a working population. There was no difference in HbA_1c between sexes. The distribution of HbA_1c was approximately normal with a slight difference between mean and median values at all ages in both sexes. HbA_1c increased with deterioration of glucose tolerance and with all the known risk factors for diabetes (age, obesity, family history of diabetes, history of a large newborn delivery); age but not body mass index appeared as a factor influencing HbA_1c independently. In women, HbA_1c levels rose particularly at the age of menopause but the use of oral contraceptives or oestrogens made no difference. In both sexes, HbA_1c was higher in smokers than in non-smokers. No consistent seasonal variation was observed. Haematologic factors had a negligible influence on HbA_1c level. HbA_1c was more highly correlated with fasting plasma glucose than with 2 h-plasma/glucose (r=0.20 vs 0.11). In a stepwise multiple regression analysis, age followed by fasting plasma glucose were the only two significant factors associated with the level of HbA_1c. These data indicate that HbA_1c is influenced only by factors closely linked to diabetes.