The relationship between blood glucose and glycosylated haemoglobin (HbAlc) had 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 2h post-prandially) were made an average of 41 times in each patient during this period and HbAlc 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 HbAlc (r = 0.69, p less than 0.001; r = 0.46, p less than 0.001, respectively). A "glycosylation index" was calculated for each patient (HbAlc divided by the mean blood glucose value). There was a significant correlation between the "glycosylation index" and duration of diabetes (r = 0.68, p less than 0.001). In contrast, there was no correlation between red cell 2,3-diphosphoglycerate and HbAlc or "glycosylation index". These findings suggest that increasing duration of diabetes influences the post-translational formation of HbAlc and that isolated HbAlc values need to be interpreted with caution in the pregnant diabetic.