There is good evidence that the non-enzymic chemical modification of proteins plays a role in the aetiology of cataract and diabetic sequelae. This paper presents new evidence that glycosylation of two major lens structural crystallins, alpha- and gamma-crystallins, by glucose 6-phosphate (G6P) induces conformational changes in the proteins. In addition the surface charge on the molecules is altered. These changes would affect protein-protein and protein-water interactions within the lens and could lead to disruption of the short-range order of the lens proteins which is essential for lens transparency. Conformational changes to lens proteins are known to occur in human cataractous lenses but their cause in vivo is not established. Cumulative chemical modification of proteins, over a period of decades, is a strong candidate as a causal agent.