Hyperuricemia is a well-known consequence of glucose-6-phosphatase (G6Pase) deficiency, the enzymatic abnormality that characterizes glycogen storage disease (GSD) Type Ia. However, acute gout as the presenting manifestation of GSD Type Ia has been reported in only a few patients. We report a new case in a 17-year-old male evaluated for acute gouty tendinitis in the right Achilles tendon. Blood tests showed chronic acidosis with high levels of uric acid, lactic acid, and cholesterol. A liver enzyme study confirmed the diagnosis of GSD Type Ia. A genetic study showed that the index patient and his sister were composite heterozygotes for the known mutation R83C and the previously unreported mutation M5R. Acute gout in an adolescent with liver enlargement and high blood levels of uric acid and cholesterol should suggest GSD. Demonstration by molecular biology techniques of a mutation in both alleles of the G6Pase gene establishes the diagnosis of GSD Type Ia, obviating the need for a liver biopsy.