A 55-year-old woman presented with a one-year history of painful muscle cramps and progressive flexion contractures of the arms, pelvic girdles and knees. Laboratory evaluation was summarized as follows: low plasma cortisol and ACTH levels, delayed response of plasma cortisol to ACTH administration, no response of plasma ACTH level to insulin administration, and normal plasma LH, FSH, GH, TSH and PRL levels. She was diagnosed as isolated ACTH deficiency. EMG was silent in contractured muscles at rest. Biopsy of the biceps femoris muscle revealed a marked reduction in fiber size, type 2 fiber atrophy and type 1 fiber predominance. The nerve conduction velocities of the peripheral nerves were found to be decreased. The biopsied specimen of the sural nerve revealed a loss of large myelinated fibers and segmental demyelination. Both flexion contractures and nerve conduction velocities were gradually improved with the replacement of hydrocortisone.