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.