Background: Danon disease is due to primary deficiency of lysosome-associated membrane protein-2. Objective: To define the clinicopathologic features of Danon disease. Methods: The features of 20 affected men and 18 affected women in 13 families with genetically confirmed Danon disease were reviewed. Results: All patients had cardiomyopathy, 18 of 20 male patients (90%) and 6 of 18 female patients (33%) had skeletal myopathy, and 14 of 20 male patients (70%) and one of 18 female patients (6%) had mental retardation. Men were affected before age 20 years whereas most affected women developed cardiomyopathy in adulthood. Muscle histology revealed basophilic vacuoles that contain acid phosphatase-positive material within membranes that lack lysosome-associated membrane protein-2. Heart transplantation is the most effective treatment for the otherwise lethal cardiomyopathy. Conclusions: Danon disease is an X-linked dominant multisystem disorder affecting predominantly cardiac and skeletal muscles.