The fluorescence of the lens has the characteristics of a life-long cumulative index of glycemia, and it is elevated in patients with diabetes in proportion to the duration of diabetes and the level of glycemia. Consequently, lens fluorometry should be capable of providing an estimate of prediagnostic disease duration when patients are diagnosed with type 2 diabetes. Among euglycemic subjects, first-degree relatives of patients with type 2 diabetes tend to have higher glycemia levels than subjects who are unrelated to diabetes patients. They should therefore also have the higher lens fluorescence values.