Several infants and neonates who had received long-term ventilatory assistance had subnormal diaphragmatic muscle mass on gross necropsy examination. We conducted a retrospective study of our hospital infant necropsy files to determine whether prolonged ventilatory support was associated with diminution in myofiber cross-sectional area selectively affecting the diaphragm. We found that long-term ventilatory assistance may predispose diaphragmatic myofibers to disuse atrophy or to failure or normal growth. This phenomenon may contribute to difficulties in weaning infants from ventilatory support.