Muscle specimens from two baby males with fatal X-linked recessive (XLR) form of central nuclear myopathy (CNM) were studied by immunochemistry using cell surface molecules (N-CAM) and by electron microscopy. In both neonates muscle fibres with morphology of myotubes strongly expressed N-CAM. The expression of N-CAM in non regenerating postnatal human muscle suggests that neonatal X-linked fatal form of CNM is characterized by arrest or delay of maturation of muscle fibres.