OBJECTIVE: To determine whether a specific subtype of glycogen storage disease type III is associated with myopathy and cardiomyopathy. DESIGN: Case series. SETTING: Three referral medical centers. PATIENTS: All patients with glycogen storage disease type III who were followed in 1990 and for whom both immunoblot analysis and clinical data were available. MAIN OUTCOME MEASURES: Evaluation for myopathy and cardiomyopathy included determinations of serum creatine kinase activity; muscle strength testing; ischemic exercise testing; nerve conduction studies; and electromyographic, electrocardiographic, and echocardiographic studies. RESULTS: Three patients with deficient debranching enzyme activity and deficient immunoreactive material in liver but normal debranching enzyme activity in muscle (glycogen storage disease IIIb) had no clinical evidence of myopathy or cardiomyopathy. Serum creatine kinase activity, muscle strength, ischemic exercise testing, electrocardiograms, and echocardiograms were normal in these patients. These studies and electromyograms were abnormal in seven patients with total debranching enzyme deficiency and an absence of immunoreactive material in both liver and muscle (glycogen storage disease IIIa) and in three patients who had debranching enzyme transferase deficiency but normal glucosidase activity in both liver and muscle (glycogen storage disease IIId). All 10 of these patients had progressive myopathy, and 6 had progressive cardiomyopathy. CONCLUSION: Clinical features of glycogen storage disease type III correlate with the particular biochemical defect seen with the disorder. Assessments of debranching enzyme or debranching enzyme transferase activity in muscle can be used to predict whether patients with glycogen storage disease type III will develop myopathy and cardiomyopathy.