A white diabetic patient on insulin therapy presented with recurrent hypoglycemia despite very high glycosylated hemoglobin (HbA1c) values. Hemoglobin (Hb) variants, chemically modified Hb, and abnormalities of red cell turnover cause errors in HbA1c measurement. Widely prevalent Hb variants affecting HbA1c estimation include HbS and HbC in African Americans, HbE in southeast Asians, and carbamyl-Hb in uremic patients. In addition, there are at least 893 other Hb variants as of 2005, many of which affect HbA1c estimation. HbA1c values are also affected by methodology of estimation. Our patient had HbJ, which is rare amongst whites. The relationship between HbA1c values and mean plasma glucose allows estimation of expected HbA1c. Significant discrepancy between expected and measured HbA1c should be evaluated. Considering Hb variants, evaluating for the same and estimating HbA1c with the appropriate method under such circumstances are described. Numerous new or rare Hb variants will be diagnosed if suspicion is appropriately entertained.