Fifteen patients with chronic sensory ataxia caused by a large-fiber sensory neuropathy were studied and followed up for a period of 17.4 years (range, 4 to 41). When first seen, they had distal paresthesias and sensory ataxia of slow onset and progression, areflexia, normal strength, and a profound loss of proprioceptive and kinesthetic sensation extending up to the most proximal joints. Needle electromyogram and motor-nerve conduction velocity findings were normal in most of the patients and sensory potentials were absent in all. Nerve biopsy showed severe loss of the large myelinated fibers. Nine patients had a serum monoclonal or polyclonal gammopathy (3 with IgM kappa, 1 with IgA kappa, and 5 with a polyclonal increase of IgG, IgA, or IgM), and 8 had elevated cerebrospinal fluid gamma globulin levels in spite of low normal total cerebrospinal fluid protein levels. No circulating antibodies to ganglionic neurons were found. Therapy with immunosuppressants or plasmapheresis was unsuccessful. All patients are disabled and their conditions have continued to worsen without signs of malignancy or systemic illness during a mean follow-up period of 17.4 years. Chronic idiopathic ataxic neuropathy is a proprioceptive neuropathy, clinically indistinguishable from the one associated with carcinoma or pyridoxine abuse due to involvement of the dorsal root ganglia, and could represent a distinct form of an indolent, slowly progressive sensory neuronopathy (ganglionopathy). Although immunopathological mechanisms may play a role, especially in patients with an associated paraproteinemia, the resistance of such patients to therapy, the progressive course, and the resemblance of this disorder to other toxic neuronopathies associated with pyridoxine abuse or doxorubicin administration suggest a possible toxic etiopathogenesis.