Renal hyperfunction and hypertrophy are characteristic findings in the early diabetic state, both in diabetes mellitus and in experimental diabetes. A number of structure-function relationships and their likely mechanisms are discussed. The metabolically induced hypertrophy of glomerular capillaries possibly plays a central role. Its cause is not known, but recent results on its time-course emphasize the probable long-term consequences of the irreversibility of the accumulation of basement membrane material.