31Phosphorus magnetic resonance spectroscopy allows examination of skeletal muscle bioenergetics in vivo. The forearm muscle of four male patients receiving regular blood transfusions for myelodysplastic anaemia showed increased acidification and phosphocreatine depletion during exercise (compared to six age-matched male controls). Transfusion produced no significant improvement in these measurements or in the estimated maximum oxidative capacity of the muscle. We conclude that transfusion of patients with chronic anaemia (haemoglobin of 8-9 g/dl) does not improve skeletal muscle oxidative metabolism.