Eleven sural nerve biopsies from nine acromegalic patients, with and without peripheral neuropathy, were examined utilizing routine sections, single teased fibres and electron microscopy. The initial basic pathological lesion in both groups consisted of demyelination combined with hypertrophic formations affecting the Schwann cell system of the small diameter fibres. Second biopsies from two patients with neuropathy demonstrate the natural history of the lesion. The progression to eventual end-stage neuropathy is documented and is due to the marked onion bulb formation. Recovery is unlikely. Our findings indicate that the development of peripheral neuropathy in acromegaly is a serious complication. Deterioration, both clinically and pathologically, can be expected if the growth hormone remains persistently elevated.