To examine the temporal relationship between Hb A_Ic values and various indices of blood glucose control, 38 diabetic and 28 nondiabetic youth counsellors employed at two summer camps for diabetic children took part in an eight-week study. Each week fasting determinations were made of Hb A_I, Hb A_Ic, serum cholesterol, triglyceride and growth hormone and plasma glucose. Total daily urine glucose excretion was measured approximately two times per week, capillary glucose values were measured fasting and at 11 a. m. and 3 p. m. on two days per week, and urine glucose was measured semi-quantitatively four times per day. As Hb A_I was correlated highly with Hb A_Ic (r = 0.997), it was used as the primary index of glycosylated haemoglobin. The mean values of Hb A_I, serum cholesterol and triglycerides and fasting plasma glucose were all significantly elevated in the diabetic group but only Hb A_I values provided total separation of the two groups. Within the diabetic group the Week 8 Hb A_I values showed a significant correlation with the Week 8 mean capillary glucose concentrations, the proportion of urine tests showing 2% and 0% glycosuria, and mean serum triglycerides. Correlations of Week 8 Hb A_I with the mean values of these glycaemic parameters for each week of the study demonstrated low order correlations with the glycaemic measures of Week 1, and a progressive increase in the degree of correlation reaching a plateau with the glycaemic measures of Week 4 to 8. Similar correlation analysis using the Hb A_I values from Week 4 confirmed these findings. Therefore, while Hb A_I provides an index of the control of diabetes, it appears to be more acutely responsive to blood glucose alteration than generally recognized.