In a 54-year-old male a severe sensory neuropathy was observed during treatment for pulmonary tuberculosis with isoniazid (400 mg/day) and pyridoxine (600 mg/day). Eight months after withdrawal from isoniazid the sensory symptomatology was still progressing, although muscle strength was never reduced. A sural nerve biopsy revealed marked loss of large myelinated fibres. Only when pyridoxine treatment was interrupted did a slow improvement begin. A clinical and electrophysiologic follow-up showed a very slow and still incomplete recovery after four years. The possibility of an unusual individual susceptibility to toxic effects of pyridoxine is considered.