The present studies examined the relationship between fasting blood glucose and Hb A in C57BL/6J, DBA/2J, and KK/HlJ mice with and without diabetes mellitus. Daily averaged blood glucose levels based on continuous glucose monitoring and effects of 6-h vs. overnight fasting on blood glucose were determined. Daily averaged blood glucose levels were highly correlated with Hb A, as determined with a hand-held automated device using an immunodetection method. R values were 0.90, 0.95, and 0.99 in KK/HIJ, C57BL/6J, and DBA/2J, respectively. Six-hour fasting blood glucose correlated more closely with the level of daily averaged blood glucose and with Hb A than did blood glucose following an overnight fast. To validate the immunoassay-determined Hb A, we also measured total glycosylated hemoglobin using boronate HPLC. Hb A values correlated well with total glycosylated hemoglobin in all three strains but were relatively lower than total glycosylated hemoglobin in diabetic DBA/2J mice. These results show that 6-h fasting glucose provides a superior index of glycemic control and correlates more closely with Hb A than overnight-fasted blood glucose in these strains of mice. Copyright © 2008 the American Physiological Society.