Barth syndrome is an X-linked cardiac and skeletal mitochondrial myopathy. Barth syndrome may be due to lipid alterations because the product of the mutated gene is homologous to phospholipid acyltransferases. Here we document that a single mitochondrial phospholipid species, tetralinoleoyl-cardiolipin, was lacking in the skeletal muscle (n = 2), right ventricle (n = 2), left ventricle (n = 2), and platelets (n = 6) of 8 children with Barth syndrome. Tetralinoleoyl-cardiolipin is specifically enriched in normal skeletal muscle and the normal heart. These findings support the notion that Barth syndrome is caused by alterations of mitochondrial lipids.