We encountered 2 patients with hereditary motor and sensory neuropathy (HMSN) type I who had marked weakness developing during several months superimposed on chronic peroneal muscular atrophy. Further studies disclosed a chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) in one patient and CIDP associated with polymyositis in the other. Both patients responded to prednisone and azathioprine with substantial improvement. Patients with HMSN who develop rapid progression of weakness should be evaluated for superimposed, potentially treatable dysimmune neuromuscular disorders.