A girl presented with an important growth retardation, hepatomegaly, fasting hypoglycemia, lactic acidosis, increased serum cholesterol, triglycerides and uric acid, and increased liver glycogen (7.5%). There was no rise in blood glucose after IV galactose or fructose, but glucagon gave a delayed response. Type Ib glycogen storage disease was suggested by the low normal activity of glucose-6-phosphatase (G-6-Pase) which reached 1.8 units/g (normal, 2 to 10 units/g) and the normal activity of other glycogenolytic enzymes, measured in homogenates prepared in H2O (mean +/- S.E. in control subjects: 59% +/- 7; in type Ia GSD: 92% +/- 3). The activity of G-6-Pase measured as described above increased to 3.8 units/g of liver 1 year after PCS and 7.85 units/g of liver after 3 years. At that time, a simultaneous assay of the enzyme in a fresh, previously not frozen liver biopsy, homogenized in 0.25 M sucrose, revealed only about 29% of the activity of the same sample prepared in H2O (mean +/- S.E. in three controls: 95.8% +/- 8.9.