Deficiency of the enzyme glucose-6-phosphatase is the biochemical defect in glycogen storage disease type I (GSD I). Normally this enzyme is present in the liver, intestine and kidneys. The lack of the enzyme in the kidney makes it obvious that glycogen storage will not be restricted to the liver but that also the kidneys will be involved, possibly resulting in renal damage. Glycogen storage in the kidney is most outspoken present in the proximal tubular cells. In case of insufficient metabolic control, a Fanconi-like syndrome can develop, disappearing with improved therapy. Although renal disease has not been considered a problem in GSD I, recent findings indicate that especially in adult patients chronic renal disease is a common complication. In the past gout nephropathy and renal stones were the complications mentioned. Recently it appears that in a considerable number of patients after a period of 'silent' hyperfiltration, renal damage develops with proteinuria, hypertension and renal dysfunction later on. In biopsies of such patients focal glomerulosclerosis is found.