Lysosome-associated membrane protein-2 deficiency (LAMP2 deficiency), or Danon disease, is a rare X-linked lysosomal disease characterized by cardiomyopathy, vacuolar myopathy, and mental retardation. Less than 20 families with mutations of the Lamp-2 gene have been reported. We describe a family from Sardinia with eight affected patients (4 females and 4 males) and a novel mutation in exon 2 of the Lamp-2 gene (c.102_103delAG). Females developed isolated cardiomyopathy in adulthood, whereas males presented with cardiomyopathy, myopathy, and mental retardation before the age of 20 years. Cardiomyopathy was lethal in three females in their 40s and in three males before the age 20 years. One patient was su ccessfully treated by heart transplantation with more than 5-year follow-up. T his study demonstrates that Danon disease is a frequently fatal condition that i s potentially treatable with heart transplantation. Lysosome-associated membrane protein-2 deficiency (LAMP2 deficiency), or Danon disease, is a rare X-linked lysosomal disease characterized by cardiomyopathy, vacuolar myopathy, and mental retardation. Less than 20 families with mutations of the Lamp-2 gene have been reported. We describe a family from Sardinia with eight affected patients (4 females and 4 males) and a novel mutation in exon 2 of the Lamp-2 gene (c.102_103delAG). Females developed isolated cardiomyopathy in adulthood, but males presented with cardiomyopathy , myopathy, and mental retardation before the age of 20 years. Cardiomyopathy was lethal in three females in their 40s and in three males before the age 20 years. One patient was su ccessfully treated by heart transplantation with more than 5-year follow-up T his study demonstrates that Danon disease is a frequently fatal condition that is potentially treatable with heart transplantation.