OBJECTIVE- HbA(1c) is a standard clinical assessment of glycemia and the basis of most data relating glycemic control to complications. It remains' unclear, however, whether HbA(1c) is affected by glycemic variation and mean glycemia. RESEARCH DESIGN AND METHODS- To test this question, we analyzed the statistical relationship between. HbA(1c) levels. and glycemic variability as measured by self-monitoring of blood glucose (SMBG),. The records of 156 subjects were studied. SMBG data for the preceding 3 months. were downloaded, and HbA(1c) was measured by ion-exchange high-performance liquid chromatography. Simple- and random-effects linear regression models were used to assess the independent contributions of mean blood glucose (136) and SD of BG to HbA(1c), after adjusting for the mean BG. RESULTS- Mean +/- SD for HbA(1c) was 7.66 +/- 1.11% and for BG was 8.5 +/- 1.9 mmol/l (153.3 +/- 34.9 mg/dl); SD of BG for individual subjects was 3.5 mmol/l (63.3 mg/dl), varying from 0.4 mmol/l (8.1 mg/dl very stable glycemia) to 8.4 mmol/l (152.5 mg/dl; very unstable glycemia). A close correlation between mean BG and HbA, was demonstrated (r = 0.62). Also within-subject SD of BG correlated with HbA(1c) (r = 0.375), indicating that people with poorer glycemic control had higher BG variance. After adjusting for mean BG in a linear regression model, however, the effect of the within-subject SD of. BG on the HbA(1c) was insignificant. Several further analyses confirmed the strength of the observation. CONCLUSIONS- HbA(1c) reflects mean glycemia and is not meaningfully affected by glycemic instability after adjusting for mean BG.