Segmental muscle enlargement occurs in a variety of neurogenic conditions. We present a case with calf hypertrophy, likely produced by partial denervation and continuous neuromuscular irritability, which was caused by a tethered spinal cord that was demonstrated by MRI. Muscle MRI correlated with muscle biopsy findings in which atrophy and hypertrophy were accompanied by rimmed vacuoles. (C) 1994 John Wiley and Sons, Inc.