BACKGROUND: Phosphoglycerate mutase (PGAM) deficiency (glycogen storage disease type X) has been reported in 12 patients of whom 9 were African American. OBJECTIVE: To describe 2 patients, 1 of Pakistani and 1 of Italian ethnic origin, with typical clinical and biochemical changes of glycogen storage disease type X and novel mutations in the gene encoding the muscle subunit of PGAM (PGAM2). DESIGN: Clinical, pathological, biochemical, and molecular analyses. SETTING: Tertiary care university hospitals and academic institutions. Patients A 37-year-old Danish man of Pakistani origin who had exercise-related cramps and myoglobinuria and a 65-year-old Italian man who had exercise intolerance and myalgia but no pigmenturia and had undergone long-term statin therapy. MAIN OUTCOME MEASURES: Clinical course and biochemical and molecular features. RESULTS: Biochemical evidence showed severe isolated PGAM deficiency, and molecular studies revealed 2 novel homozygous mutations, a nonsense mutation and a single nucleotide deletion. Pathological studies of muscle showed mild glycogen accumulation but prominent tubular aggregates in both patients. CONCLUSIONS: We found that glycogen storage disease type X is not confined to the African American population, is often associated with sarcoplasmic reticulum (SR) proliferation, and is genetically heterogeneous.