The Diabetes Control and Complications Trial (DCCT) demonstrated that intensive therapy aimed at improved glucose control markedly reduced the risk of diabetes complications compared with conventional therapy. The principal determinant of risk was the history of glycemia. Recently, McCarter et al. (Diabetes Care 27:1259-1264, 2004) have presented analyses of the publicly available DCCT data using their hemoglobin glycation index (HGI), which is computed as the difference between the observed HbA1c (A1C) and that predicted from the level of blood glucose. In their analyses, the HGI level was a significant predictor of progression of retinopathy and nephropathy in the DCCT, which the authors claimed to support the hypothesis that the biological propensity for glycation, so-called biological variation in glycation, is another mechanism that determines risk of complications. However, we have criticized these analyses and conclusions because, from statistical principles, the glycation index must be positively correlated with the A1C level and thus may simply be a surrogate for A1C. Herein, we present the statistical properties of the glycation index to document its high correlation with A1C. We then replicate the analyses of McCarter et al. using both the HGI and the A1C together. Analyses show conclusively that the glycation index is not an independent risk factor for microvascular complications and that the effect of the glycation index on risk is wholly explained by the associated level of A1C. The HGI should not be used to estimate risk of complications or to guide therapy.