An X-linked recessive disease is reported in a large pedigree. The disease is characterised by a triad of dilated cardiomyopathy, neutropenia and skeletal myopathy. The untreated patients, all boys, died in infancy or early childhood from septicemia or cardiac decompensation. Ultrastructural abnormalities were observed in mitochondria in cardiac muscle cells, neutrophil bone marrow cells and to a lesser extent (0-9%) in skeletal muscle cells. Membrane-bound vacuoles were seen in neutrophil bone marrow cells. Intramuscular fat droplets were increased in type I skeletal muscle fibres. An affected patient had intermittent lactic acidemia, borderline low plasma carnitine, the latter decreasing during periods of illness, and low muscle carnitine (27% pretreatment; 35-40% posttreatment). While on treatment with oral carnitine he had less weakness and no cardiac complaints, but his neutropenia was not affected. Respiratory chain abnormalities were observed in this patient's isolated skeletal muscle mitochondria. These were: (1) diminished concentrations of cytochromes c1 + c, b and aa3 to 29, 47 and 64% of the averaged controls, and (2) a lowered P:0 ratio for oxidation of ascorbate + TMPD, with diminished uncoupler stimulated Mg2+-ATPase activity. Muscle AMP deaminase was deficient (5 resp. 17%). Only one previous report (Neustein et al. 1979) on X-linked mitochondrial cardiomyopathy exists, which probably refers to the same entity. Biochemical studies and haematological abnormalities (neutropenia) are reported for the first time.