Danon disease is an X-linked dominant multisystem disorder that includes hypertrophic cardiomyopathy with skeletal myopathy, and results from mutations in the gene encoding the lysosome-associated membrane protein-2 (LAMP-2). To date, over 20 different mutations in LAMP2 have been identified. Three members of a family, a male proband (18 years old) and 2 sisters (15 and 20 years old) were studied. Their mother had been diagnosed with dilated cardiomyopathy at the age of 39 years, and died from advanced heart failure at the age of 43 years. The proband developed marked concentric hypertrophy at the age of 5 years and DNA analyses revealed a novel hemizygous frameshift mutation (c.573delA) in exon 5. The 2 affected sisters were also heterozygous for the same mutation. Functional analyses of this novel LAMP2 mutation are mandatory.