A prospective study of 14 patients (ages 6 months to 33 years) with glycogen storage disease, Type I (GSD-I) was carried out in order to define the character and frequency of renal dysfunction. A marked increase in the glomerular filtration rate (GFR) was documented in virtually all subjects, with the mean GFR raised by approximately 50%, to the range of 170 ml/min/1.73 m2. While this constituted the only renal abnormality found in the younger patients, a significant increase in urinary albumin excretion was seen in three teen-aged individuals; three patients over 20 years of age exhibited frank proteinuria (2 to 8 g/day). Renal biopsy on two of the proteinuric subjects revealed focal and global glomerulosclerosis and interstitial fibrosis. Evaluation of factors known to cause an increase in GFR did not define the precise etiology for its elevation in GSD-I. These studies suggest that: (1) glomerular damage and chronic renal disease are common in older patients with GSD-I; (2) the renal injury appears to be specifically related to GSD-I and is not secondary to the treatment of the disease; and (3) the natural history of the renal lesion in GSD-I may be analogous to that seen in insulin-dependent diabetes, with a "silent" period where hyperfiltration is the only demonstrable renal abnormality, followed by evidence of increasing glomerular damage progressing from microalbuminuria to frank proteinuria.