The relationship between blood glucose and glycosylated haemoglobin (HbA_1c) has been investigated during an 8 week period in 53 Type 1 (insulin-dependent) diabetic women studied during the third trimester of pregnancy. Blood glucose estimations (fasting and 2 h post-prandially) were made an average of 41 times in each patient during this period and HbA_1c was determined once at the end of the study. There was a significant correlation between both the mean blood glucose over the preceding 8 weeks and the standard deviation of the fasting blood glucose with HbA_1c (r=0.69, p<0.001; r=0.46, p<0.001, respectively). A glycosylation index was calculated for each patient (HbA_1c divided by the mean blood glucose value). There was a significant correlation between the glycosylation index and duration of diabetes (r=0.68, p<0.001). In contrast, there was no correlation between red cell 2,3-diphosphoglycerate and HbA_1c or glycosylation index. These findings suggest that increasing duration of diabetes influences the post-translational formation of HbA_1c and that isolated HbA_1c values need to be interpreted with caution in the pregnant diabetic.